I decided to take a quick look at this and see if I could fix the error at least. When I tested it on my local machine there was no error, but I'm running a newer version than the release so I looked and it turns out I did recently already fix the error, but it was after the new release.
You can fix it on your installation if you edit the stored procedure: mp_ForumPosts_SelectByThread
The problem was caused by null being returned for the user post count when the user doesn't exist. You can fix it by changing this:
'PostAuthorTotalPosts' = up.TotalPosts,
to this:
'PostAuthorTotalPosts' = COALESCE(up.TotalPosts, 0),
this will convert the null to 0 and prevent the error. Its still a very poor implementation, the name of the anonymous user is hard coded to "Guest" in the stored procedure so its not even localizable. As I mentioned before, since there is no captcha implemented in the forum, allowing anonymous posts is going to invite a lot of spam, so its still porrly implemented even with this fix.
Hope it helps,
Joe