aperently the calendar didn't liked a repeating ICS-calendar entry.
I had the 'bad' calendardb file copied it to my pc and used a sqlite browser to browse thru in the hope to find what was wrong, found an more then 10000 times repeating in the Parameters table. after removing that entry (id) from all the other tables too the db seems ok. copied it back to my n900 and restarted the phone.
calendar seems ok again.