A couple of people registered for my new wpForo forum. When they tried to make a first post they got a posting error. The post title was visible, but when opening the post page it doesn't show the post content. When they tried to make another post it worked fine. Both people had exactly the same problem with their first post. Why is this?

I checked the Forum Tools --> Debug --> Tables. It says "No problems found in database".

I checked the Forum Tools --> Debug --> Errors & Issues. It says "No issues found". The Error Logs is empty.