Had to deal with a weird problem and finally found the solution. So I wanted to post it here in case it will help somebody else.

Symptoms:
When logging into KDE with a correct user and password combination the screen goes blank for a second and re-shows the login-screen.
Upon second login with same credentials all goes black and system locks up completely. No X-Server, no tty's.

Problem:
I ran out of disk space because of reformatting videos (a lot of) using ffmpeg through kdenlive. So it simply filled my HDD to the rim.

One would not think, that this would cause so much trouble and disable the complete graphical desktop environment. Plus it was a bit of search to figure the source of the problem. I think I have to go and get an additional HDD to do all the video conversion stuff on.

Analysis:
In order to find out I waited for the KDE login screen to appear but instead of logging in switched to a console which worked fine. So I had root and user access to the system via console.

Logging in with the user I did:
Which produced an output stating "unable to start xserver no disk space".

Solution:
I deleted any superflous files from my user account which were no longer needed, e.g. the source video files etc.

Hope that this may help somebody out there running into the same problem.

One "best practice" that I use is to not use a single partition for your system, but have separate partitions for the system and user files. this way, when the user fills up their space, the OS does not crash and burn... I also sub partition the system into various partitions in case logs, or tmp files, or misbehaving apps start filling up the disk...