I first reported this error to kde.org as I thought it belonged to Kaffeine, but it turned out that the crash is caused by Xorg.
To summarize what happened: Watching a (selfmade) video in kaffeine resulted in an immediate logout from the kde session. The crash can be reproduced.
Steps to reproduce:
1) close/save all open files
2) download this video:
I made it with paraView. The video itself is fine and can be played with any other player (e.g. vlc) or even in firefox without any problems.
3) open the video with kaffeine and play a couple of seconds
4) find yourself at the login promt
I also can also provide the Xorg.0.log.old file.
(direct link to the upload I already made in the kde.org forum)