I am experiencing exactly the same problem. I’ve set up wordpress (3.3.2 de) one or two weeks ago, bbpress last sunday. But I’ve nothing migrated.
In the role of a webmaster I can view and edit everything in the forum. During the first tests I changed the visibility to private and back. As a contributor I can only post a new entry. But I can’t even view my own posts. This was on the first day. I deactivated bbpress.
Today I reactivated bbpress. Forums and posts seemed to have gone. I set up a new forum and moved the old posts (topics) to it. With/without allowing anonymus posting everything worked fine for the contributor’s role.
But after setting the visibility to “private” (later to “hidden”) and than back to “public” the same error occurs: contributors can see the forum from the outside but none of the topics/posts.