As of Kubuntu 11.4 I experience X server crashes. The situation hasn’t changed after 11.10 release. For example, today I’ve experienced 3 unexpected crashes of X server (at that time Strigi was indexing).

I examined them and found out that in .xsession-errors there are Nepomuk errors. I would like to know what causes them.

Here is the log:
http://paste.ubuntu.com/723042/

Well, a simple way to test it is to disable Strigi indexing for a while and see if the crashes still occur.
Personally I don't believe that it's Nepomuk/Strigi that causes the crashes, but I don't have any evidence to back that up.