My previously mentioned solution for the steadily growing .xsession-errors didn't work out. It gave me some more time to take breath - but it was no sustainable solution. .xsession-errors still grew at astonishing rates, mostly due to inotify commenting every single file-change there.

My bro had the same problem and due to his pragmatics and fatalism immediately recognized and implemented the definitive solution. I now decided to do the same:

# ln -s /dev/null ~/.xsession-errors