My apologies it's a 500 error, the 404 is because I have not created a 500 error page.
I don't have any SEF running, But the 550 error comes when trying to load this page:
../index.php?option=com_comprofiler&task=pluginclass&plugin=cbgroupjive&action=groups&func=save&cat=40&grp=41&Itemid=53
Joomla version is 1.5.23
Not sure if that helps, I can't see anything else wrong, I guess I need a default 500 page before the debug mode will work.