Sorry I managed to move this to the wrong forum, or Moodle moved it withough me doing anything other than scrolling the move menu list.
I've asked Chris to move it to the Plugins Forum. so hopefully he will do this as soon as he gets my message.
It may be easier if you posted this again the the plugins forum, as the problem is indeed the Certificate plugin itself.
Have you checked out the New Plugins directory?
Hope this helps?