@Reception123 I already checked the permissions and the *-group already has the "edit" permission - that user group always had the edit permission. However, they still can't edit.
I also checked the extension and disabled the "Edit Subpages" extension (I thought it may cause this), but it didn't help either. That's why we don't know what to do anymore :-/

So, it does work now to create new namespace. However, since I am able to create new namespaces, a new bug have appeared that everyone on the wiki have. We can't use the visual editor anymore. It stopped working around the time you wrote that I can now create name spaces. I'm not sure if it really has something to do with it or if it's just a weird bug we all are experiencing though.
I already checked the settings, the visual editor is turned on.