Hi everyone,
I'm working on a Ubuntu machine. It was a fresh install about two years ago and updated till 16.04.

From one day to another my lock screen started making problems. Normally when i leave the room i lock my computer using super+L and get a password prompt to re-login.

Since two or three days, when pressing super+L the screen turns black and when it comes back up the only thing i see is the thin bar on top of the screen (where you have date and time). Its impossible for me to login from this point, i can only switch via ctrl+alt+F1-F6 and restart unity.

I tried some things recommended online, reinstall ubuntu-dektop, (re)install lightdm...
At the moment i used tweak tools to disable lockscreen. When i press super+L now, the screen looks the same but moving the mouse or pressing a key brings me back to my desktop.

I hope any of you got an idea, it's nothing huge but really annoying.

If you need more information, please ask.

Thanks!

Edit:
i created a second user and everything works fine. But if possible i'd like to keep my user instead of copy everything from A to B and configure Mailclient, etc.

Edit:
I just realised that Shift+Insert also doesn't work anymore. No idea if it's the same reason.

Given the info you posted about a fresh user working ok, personally I would create a fresh user, copy over the home personal config directories to the new user (using a binary search method i.e. half the directories, then half-again etc.), testing each time until the problem replicates itself. That way you should be able to pinpoint which config directory is causing the problems and, from there, hopefully figure out the root cause. Of course if you can put some smart guessing into the binary search then all the better.

Thank you for your answer hydrurga. I tried your solution. I copied the first half of data to the new user did a restart and the lockscreen for this user still worked. I tried to change to my main user but wasn't able to login. After typing my password and pressing enter nothing happened. Password field got cleared but it won't login.
I have no Idea why this happened, i guess there was something more wrong with my user than i thought. Deleting the password helped to get into the account again - next surprise was all files were gone.

Problem was that the main user wasn't able to encrypt his crypted files/folder.

After these errors i decided to copy everything and create a new user - like you suggested.

So i created a new user, encrypted the files, copied everything and now it works without any problems.

Thanks for getting back on this, Effman. Wise decision - it prevents other unforeseen problems arising in the future. Sometimes it's interesting to find out why a problem occurred and try to resolve it, but the most important thing is to have a working system.