Deltajo, Are you going to use PPC with the ubb.threads package or will it eb a standalone or use another bulletin board? It apepars this problem is due to a break in communication when trying to set it up to use the 'threads userbase instead of it's own. With the ever changing code and updates of other non photopost products, it can be difficult to tie up all the loose ends. That coupled with 'threads being out in beta form (current release is 6.5 b4) can throw a wrench into the works. I'm a beta tester for threads and I know how much changed since the last release.
If you need help with the install you can contact me privately at firstname.lastname@example.org