Ubuntu: no X11 and/or Login loop ?!

[^rooker]

[PROBLEM]
A friend of mine couldn't login to her computer this morning, because the graphical interface X11 never came up after boot.
There were some boot messages in text-mode left on the screen. No error messages, however.

I tried the text console (Ctrl+Alt+F1) and could login without problems. So I knew that the OS itself was fine.
Then I tried to start X11 under her user:

Code: Select all

$ startx

But it failed. I must admit I should have checked the logs in ~/.xsession-errors, but I forgot.

However, in the end I tried "startx" as root and it worked.
That's when I realized that actually the disk was full.

I freed some space, rebooted and tataa: The X11 login screen popped up. YAY!

Now there was another problem:
When trying to login under her user, the screen showed some text console messages and then reverted back to the login prompt.
A login prompt loop!

[SOLUTION]
First of all: Next time check ~/.xsession-errors before doing anything else!
In my case, it explained why it wouldn't let me log in to X11: