Ok, I have found the culprit (it took me about 2 hours of mvoing directories in and out, and restarting KDE),
In my home I had a folder: ~/.local/share/mime
Obviously, there was something in it, that made KDE4.2.95 go ballistic. Now I have renamed it and I can again log in as before and everything looks nice and shiny.
The only question is, what exactly is "~/.local/share/mime" good for, and what might I now be missing (but I suspect, that it is nothing extremely important
) ... and yes, I know what MIME is, just don't know,what component/application exactly depends or uses this directory.
Well, however, if someone has more thoughts on this, let me know.