After reading
www.joomlapolis.com/forum/147-potential-bug/174808-internal-server-error-with-cb-17?limit=6&start=24#183740
I can add:
- I am not upgrading, I'm staying on 2.5.x all the time.
- I made some changes in usergroups lately, I cannot say if it was exactly same moment the error started to appear
- I tried to delete all usergroups and accesslevels I created but didn't help
- I looked into mysql database and all records in user related tables seem to refer to existing records...
- I deleted usergroup 6 (which was "Manager" and was there from the beginning) it started to work
So, since the crash was on line 497 which is
$paths[$gid]=array_merge( $paths[$gid], $this->get_group_parent_ids(
6 ) );
and just deleting group 6 helps, there is something to think about.