Hmm I’m far far far from qualified to really help on this, but I can do my best.
Assuming you kept all the defaults, the forums should be being managed through the bbpress plugin. I haven’t digged around in that code much, so I’m not sure where to point you farther than that.
In my (very limited) experience, a totally blank page usually means some kind of fatal PHP error- a missing semi-colon, undeclared variable, etc., so it might even be worth running the relevant .php files through the basic php debugger.
Did you do anything special to the forums? Add on any extra functionality through a plugin or something similar?
Sorry I can’t be of more help!