Well as a guest I can see the forum summary, I can see the forum topic headers. But when i select a thread I get the same thing no thread.
I registered as rer.
I assume the default group is USER.
Now it looks like the board is closed to the USER group because I get the message that there are no forums to view.
If I log out I am back to what I saw before. So I think it is a permissions issue.
I see two options. Completely go through every setting for forums and groups.
You could just pick one forum and one group. Check what group I am a member of.
Or open a support ticket to fix the problem.
I am not sure what you upgraded from but I have seen issues with upgrading from threads 6 to 7 where most everyone is imported as a moderator instead of a user. I think Rick has a simple query to fix that issue without loosing the real moderators.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers