DR3, that is a problem with your who's online script. You need to fix the SSI call in ubb_library2.pl. Check the who's online thread for multiple answers to that question.
Nick, quite possibly a bug.... I haven't seen it, but doesn't mean it isn't there... I have seen it autodelete without the 500 ISE... I know you said your file permissions were correct, but is your private directory (and the files within) set to 777?