Okay, I have installed pastebin it and my .xsession-errors output is at http://paste.ubuntu.com/1334562

The references to the "Percy Faith Orchestra" is a folder of MP3s which used to be contained on the Desktop, but is now in my MP3 directory. However, there are other folders of music and PDFs also stored in the Desktop directory that are not mentioned in the xsession-errors output.

I have my /home directory in a separate partition, which needs to be referenced as "home" in fstab when I log on. So what I did rather than create a new user, was to comment out that line in fstab so when I log on, I just get a default user home directory, rather than having /home reference my home partition.

When I did this, and rebooted, my login produced a correct Unity screen, ready to use. And I could access the files in my /home partition, but only as a separate partition, not as /home. And, of course, all my applications have their preferences reset to the default. So this does prove that the problem is a setting inside my existing proper /home directory.

Before everything went wrong, I used the LXDE desktop as my environment, as I don't like Unity much. However, when I booted into this with my "default" /home directory, although it went in, it immediately started chucking up a load of errors, saying it "needs to close" and would I like to send a report to Canonical about the problem. It did this repeatedly, so I logged out and went back into Unity. This problem may be connected to attempting to use the LXDE desktop without having all the config files, which would of course be located in my own /home partition.

I am sure the solution to this problem must be located in a config file in my /home directory. Can you think of what it could be?

Thank you for your patience in helping me with this matter.

Using a non-default desktop environment is an important piece of information resolving this. In the future, please mention related details like this where you've customized the system. Using your properly mounted /home partition when you log in using LXDE as your environment your system hangs. However, when you log in using Unity you log in just fine and are able to use the system. Is this correct?

I looked at your .xsession-errors file and didn't see anything interesting. Let's see if we can find some logs to look at to figure out the problem. First, let's grep through all of your logs and figure out which files contain the term "LXDE".

When you see files which contain LXDE then inspect them and look for error messages which you think might be related to your problem. Google those errors to try to find what other people are saying. If a direct google search doesn't provide results then try to broaden your search with a more generic google search.

For example, When I was googling around for this log...
I google searched for...
That yielded no results so I added in a wildcard in an appropriate place to keep the search still relevant.
And that gave me results.

Try going through your logs and apply the same process. The problem sounds like it is LXDE related if you log in just fine using Unity.

Not quite. For the avoidance of doubt, I'll say "my home partition" to refer to the partition where all my settings and documents are stored, and the place where I believe the problem is; and "home directory" to refer to the unused home directory, stored in the root partition of Ubuntu, which I can access now that I have commented out that line in fstab.

When "/home" is referencing my home partition, where the problem is, then I cannot log in successfully to any of my desktops - not Unity 2D, 3D, LXDE, OpenBox etc. However, I can do Ctrl-Alt-F2 and use a terminal, and access my home partition through /home/steve. So at least I know the home partition is available and mounted.

If I delete that line in fstab then my default home directory, part of /, will become available again on reboot; since I have never used this directory because I edited fstab upon installation of Ubuntu 12.04, the default home directory is a sparse place with none of my relevant config directories or settings. However, it also does not have the alleged "problem file(s)", and so loads Unity or LXDE up with no problems. However, upon loading LXDE, it does start putting up error messages. This may be because my home partition was being used when I installed LXDE, so this will be where all its necessary config files would have been written on install.

So without locating and fixing the issues with the alleged problem files in my home partition, I will lose all the settings for all the installed programs; unless I copy across their relevant config directories into the /home directory. This is still no good, since I then lose the use of the separate home partition, which was the point of having it in the first place. If I thought it would do any good, I would uninstall LXDE to see if that removes the problem; however, using my home partition it will not display Unity either. Although, mysteriously, it does present a mouse arrow (on a blank screen) that responds to the movements of the mouse.

So that's my position thus far. I will run your suggested searches tomorrow when I'm back home and see if anything comes up. I have Googled around and found numerous "blank screen on login" problems with Ubuntu, but these all seem to be linked to proprietary graphics drivers like NVidia not being present. My graphics are Intel and don't require extra drivers. Plus, a lot of other users' problems are system-wide and they can't log in to anything without getting a blank screen.

My suspicions are still that the updates caused the problems. Googling reveals that Ubuntu keeps a list of updates in a log at /var/log/apt/history.log, so tomorrow I'll have a look there and see if any updates to graphic capabilities have been applied.

Do you know of any log files where problems that cropped up *after* login would be written? Sometimes my login screen is set to a different resolution to my desktop once I have logged in, but I don't worry about it since the resolution on my desktop is okay once login is complete. I can only assume that this desktop resolution is stored in a file somewhere in my /home directory's config files...

The only log files I can think of looking in would be the Xorg log files in /var/log/*. Though here's a thought. You can list the open file handles when the system is hung after login. Log into the system so that it freezes up and then Ctrl+Alt+F2 to get to a terminal. Once at the terminal run...
There might be a settings file which the system hangs on parsing. If so, then it is likely the file handle would remain open during the freeze. Now that you have a list of open files (possibly configs?) then analyze the error logs associated with those applications. It may reveal the source of the problem.

SAM

Well, I did that. The files that kept cropping up were:

/.config/dconf/user
/.cache/dconf/user
and .xsession-errors

The first two are not plaintext files so I can't open them. The icon shows zeroes and ones so I guess these are binary files?

I should actually say that my system hasn't "hung" completely because the mouse pointer still moves. So it's difficult to know whether the system is logging an "error", or just doing what it is being told. Is it possible that the system - to its own satisfaction - is loading up a desktop but I am not seeing it, just a black/grey screen? I notice that when I go to the console, the text is very small. Could it be that the screen resolution is being set too high by the system? But if so, the pointer would be very small as well wouldn't it...

You said you found nothing of interest in .xsession-errors, but what about the lines at the top that say:
Are these not relevant? I wonder if the condition is caused by the desktop, or Gnome, or Unity or whatever, not starting because the right files can't be found for some updated package that's been installed in the updates.

I would say that these messages are taken from the latest .xsession-errors, which seems to have changed from the file I pastebinned the last time. Don't know whether this is good or bad...

There's not much you can do about the .config file yet... however you can delete all files and directories under .cache. If you do it's recommended that you restart the system as some applications aren't very good at handling the cache if it is cleared outside of the program interaction.

When you delete the .cache folder I actually recommend you just move it; that way you at least have a back up of it. Moving it to a different name is the same thing as deleting it because the programs will just recreate the cache if they're aware it was deleted.
The black/grey screen is the desktop even though it is partially loaded. It means there's something wrong or something is not properly autostarting.

Your resolution being set too high is not a problem at all. If your system resolution was set higher than the monitor support then you wouldn't see any screen at all (i.e. your monitor would look like it is off). Some fancier monitors are nice enough to tell you that the resolution is "Out of Range".

They may be relevant. Those entries are not in the original .xsession-errors file that you linked me. The first thing I would do is edit that xfce4-settings-helper-autostart.desktop file and check to see if it has a Name key. It would look something like this...
The Name key refers to the Name=value in the list above. You could open it by running the following command.
For the terminal I recommend vim or nano; in nano ^X to exit refers to Ctrl+X. ^ is terminal shorthand for Ctrl+.

No, that xfce4-settings-helper-autostart.desktop file does not have a Name key. It just says:

[Desktop Entry]
Hidden=false

... and that's it. Do I add one? And if so, what should it say?

It's a bit of a mystery since I've never installed XFCE...

Steve

First
Be sure to back up the file... Now go ahead and completely delete the file.
Now reboot (or restart X) and try logging into your system again. If that doesn't work then...

Second

Looking at other sources on the net it appears the contents of that file should be....
Before making those changes be sure to keep a backup copy of the old file. Also be sure that the command xfce4-settings-helper exists on your system.
SAM

I moved xfce4-settings-helper-autostart.desktop and xfce4-tips-autostart.desktop from the autostart directory up to the main /home directory, thus keeping copies but not in the autostart directory. Rebooted again - didn't work, still black screen.

I don't have a command xfce4-settings-helper on my system. As I say, I've never installed xfce4 to my knowledge; or if I did it was so long ago that I've forgotten, and I would have uninstalled it straight away. I have seen xcfe on live boot CDs before, and am not too keen on it.

So perhaps I can just leave those two autostart files where they are (i.e. not in the autostart directory, just backed up temporarily on my home partition).

I still can't take my mind off the two lines in the xsession-errors file that say "gnome-session[1603]: WARNING: Application 'unity-2d-shell.desktop' killed by signal". Surely a message saying that the desktop has been "killed" means that the process for displaying the desktop has been terminated by the system, which is why it isn't displaying?

Guys, I am happy to say I have managed to get into my system, not sure how or why it suddenly let me in but I think it was after installing YURI? which alllowed a remote log in. Anyway I immediately ran update manager and my system is up to date. Nevertheless there are still occasional graphics problems. The system does not want to run back up either. This is not a space issue. The problems appear to start if I use evolution mail which I now avoid. I will receive a new lap top next week so until I have transferred files to the new system I am going softly softly. I did notice that some functions are extremely fast with 12-10.
I really appreciated the doors you opened for me because I learn't quite a few things from your messages.

best regard
Gordon (yes I am a guy but I think you americans have twisted the english language and refer to guys as any gender unlike the brits.)

Well, unfortunately I have not managed to locate the source of the problem with my Ubuntu 12.04. However, as the problem seems to be linked to a file in my home partition, I have been using the system for the last few days just with the standard home directory and just copying relevant config files across from one /home to the other.

I'm staying with Unity and not using LXDE, even though I prefer the LXDE desktop, because the system puts up error messages when I use LXDE so I'm playing it safe.

It means I no longer have a reliable home partition so if/when I install an updated distro in the future I will just have to back up my root home directory and copy it across afterwards.

And I shall never update the system again! At least not with a general update. Maybe picking and choosing the updates and applying them one at a time, so if things go wrong I know which update caused it. It's such a gamble, if updates can break a system irrecoverably. It's a shame that I could not locate a solution in a reasonable time. My confidence in Linux has been dented, as I always thought the axiom "we don't know how to fix the problem, we'll have to reinstall" was a feature only of Windows.

Thank you Sam for your help. I did think that this problem may become widespread over time, as people update their systems and experience the same difficulties. But perhaps my problem did not happen as the result of an update and it was just co-incidence.

Eh, well we tried. It's not something normal for updates in my opinion but the kernel and Ubuntu packages are updated by humans like everything else. So it's impossible to be 100% bug free. My 12.04 is updated to the latest packages and I've not had the same experience. Did you try clearing your cache like I recommended in an earlier post from the files you presented with lsof? Moving the .config directory would even potentially have corrected the problem.

At any rate good luck with future endeavors and all.

Yes, I deleted the cache directory as instructed. I guess I could try temporarily renaming the .config directory, in case the problem is a file within that; then gradually adding the separate config sub-directories back into the mix; then if the system suddenly refuses to go in, I would know which directory was causing the problems. I may try that tomorrow; I've been using Unity desktop but I really did like the look and usability of the LXDE desktop that I had set up.

Anyway, thanks again. If I get any promising results from my testing, I'll make another post to this thread.