Was there a solution found for this? I am seeing this in EE 1.6.4 Build: 20080808 and ExpressionEngine Discussion Forum - Version 2.1.0 (20080710).
Confirmed beforehand that forums are enabled in general settings and that all forums are live. Tried deinstalling module and reinstalling a couple times. Can post topics as super admin or anyone for that matter.
[Mod edit: split from old thread]