A problem&solution for my own record.
Two bugs disappeared in PHPBugTracker and the reason was that the status_id field was set to a value that did no longer exist. The following sql solved it the problem but the bug is still there.
update phpbt_bug set status_id='8' where bug_id =31;
update phpbt_bug set status_id='8' where bug_id =32;
select * from phpbt_bug where bug_id='31' or bug_id='32';