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).
We will fix this for new registered users, but not for users who are already registered, or users are registered in another way (migrate oretc.). And it will be fixed in future releases.