OK, I've looked further. At least one item is a real problem (bug?/Feature?)
Ruben created an account. He magically had moderator powers, but couldn't see any forum. Apparently, if you do not have any default groups set that a person becomes a member of when they register, the system finds a group ( it appears to be random, but the random value is always the same. In our case everyone that registered became a moderator. That's not good. When I set the flag in the registration settings for all registered users to become part of the "user group" that disappeared.
I still have a problem that when a user is created, by default the default values for user_language and user_post_layout are null. When they are null, post contents don't show. When the language is set and user_post layout is set, posts contents are visible. There does not appear to be any settings anywhere that create a default value for these fields. I don't know how to fix this issue.
I still have an issue with the "guest" user.