I can't seem to stay out of trouble.
This time I managed to pull the wrong plug in the middle of an 'update world' and removed distfiles! Nothing but problems since...

Things are mostly sorted out now, but I have one remaining problem.
When I start KDE with startx, which I have done for years, I get the splash screen as usual. Once the KDE logo appears and is stable nothing happens for about a minute, after which the screen goes white with a black cursor. The icons that should be there are black rectangles with a little red X button on the left side. The mouse moves the cursor normally but cannot do anything like get a menu. Ctrl-Alt-Bksp kills it, and Ctrl-Shift-Fn works. Otherwise no response to the keyboard.

I have a second user defined, and that account has no problems.

Thinking it might be a permissions thing, I tried removing .kde4 and everything else KDE-related in my home directory for a fresh start, with no effect.

Logfiles show nothing amiss.

Where do I look next?

TIA
Jim

Strange, can only think of a few things:
Does the other user also use KDE?
Do your 2 users belong to different groups? If so, could be a permission issue.
How full is partition where /home resides?
You might have a few files named .X / .x , try deleting them (or just move/rename)
Last resort, create a new user for yourself, move your personal stuff.
Then you could copy over the configuration files & directories (those beginning with a dot) to see where it goes wrong, if you're interrested.

I need to open my eyes a bit wider. I missed .Xsession-errors, which was trying to tell me that it could not write ksycoca4 and asking if /var was full?

The problem was a corrupt /var/tmp/kdecache-xxxxxxx/ksycoca4. Any attempt to look at it produced i/o errors.
I fixed the problem by copying what I could from the directory to a new directory and renaming the old one out of the way.

Both users use KDE and each has its own group. / was 98% full.

Jim

If it was fs corruption, I advice you to pick any random livecd and check it using fsck. Unnoticed fs-errors can entirely crap your fs if they are not repaired. The fs's should be checked at startup, nonetheless. But if you want to be sure do it yourself from a livecd.