I am not sure with which update this started but there appears to be restriction on nicknames that prevents all numbers/numerical nicknames. This is fit and proper in my opinion but it does cause some problems in certain scenarios. For example I disable (via css) WPforo nickname changes on my boards which means that a member with the username 007 can never successfully edit their profile (the 007 nickname is auto created on registration) as it will always throw an error.

If using the user_nicename structure in WPforo settings, all numerical or number prefixed usernames also cause a problem between the Wordpress core user dashboard and the WPForo profile system too; the dashboard view link will try to go to either a user id if the all number username happens to match a random user id or it will simply go to the forum main page if it does not match a user id.

I propose that to prevent these issues there should be a filter in the WPForo core that disallows/prevents users registering with all numbers (007) or prefixed with numbers (007Bond).