I had a somewhat different problem with permissions in forums that I traced down to a check for read_private_data_p. This is a permission in dotLRN that is appropriately set if your user is a dotLRN user. However I was on using forums outside of dotLRN on a system that had dotLRN installed.
When I granted read_private_data_p to public my problems went away.
Not sure if its the same problem or not but its something to check.