This happens when visiting any special page. It suggests that no Default namespace is defined. By looking at your wiki as a guest, I cannot see any namespace that qualifies as Default, so this may be the problem.
In AdminCP > VaultWiki > Namespace Manager, modify the "Default" namespace and make sure it has a forum selected. If you aren't going to use the Default namespace for anything, make a welcome page or a rules page or something like that. If you somehow deleted the Default namespace, you can select any existing namespace to be used as the Default, or you can create a new one.