It took some effort, but i would like to thank you all for chipping in.
At least during the course of tracking this issue down i got to learn a bit more about the inner workings of ubb and how it all hangs together.
Within 2 hours of re-enabling user registrations i had 2 spammers register - this reminded me that during the course of the investigation i reverted to stock code and thus lost the "stop forum spam" mods.
