Well then something about your groups are off. I do not use phpBB 3 but I did write the integration so I know it works. I would need to actually see your site.
Like I show in our phpbb3 integration file the guest group is usergroup 1.
I know in our application the permissions work fine because they are handled the same way in every last integration we just pass the users usergroup to a function and then apply the permissions.
I would need ftp access admin login and url so I can debug your usergroup and category permissions. I need to know exactly what info is being passed from phpBB3.
You can send this data through the contact us link.