Users can't register anymore

I've been using CB for many years now, but somehow I managed to break it. Users are not able to register anymore. After submitting the registration, there's an error 'You are not authorized to view this page!'.

I was hardening the security of my site, hiding cb listings and pages from public access, even though there was no menuitem visible for public access.. Somewhere along the line I seem to have tightened the security too much, resulting in the error. I did reset everything to the state it was before, but still I still can't manage to get things working.

I have a (hidden) menuitem of type CB-profile which is set to public. The item-id of this menu is being used as the itemid in the url when one completes the registration (the view=saveRegistration page). That should be enough, shouldn't it?
I've turned off the anti-spoofing setting. Emptied all the caches and turned off caching.

I have a copy of my site on a subdomain on the same server. There everything works fine, so it's not a server configuration.

Anyone an idea?

Registration can only show the "You are not authorized to view this page!" (language key UE_NOT_AUTHORIZED) error if you disabled registration. Within Users > Manage > Options set "Allow User Registration" to "No" then in CB > Configuration > Registration set "Allow User Registration" to "Yes, Independent of CMS "allow registration"". That's the recommended settings to block Joomla registration access, but allow CB registration.

That setting has always been correct / the recommended setting. This was not changed.
It must have something to do with access rights to the registration (or rather: save registration) page.

There is no access rights beyond that check. The only other time that error outputs is if the are already logged in as you can't register on frontend if already logged in. Are you sure you're not just logged in on frontend and backend at same time when doing your tests as Joomla has a shared session feature you can toggle on/off in Joomla global configuration under the System tab.

It was reported bij someone who wanted to register for new account, so he couldn't have been logged in. I've tested it myself with different browsers, where I was not logged in either in the frontend or backend of the site. As suggested I did toggle on and off the user registration option in the Joomla user settings. Alas, still the error occurs.

I've deleted both the registration and the user profile menu item and recreated them. Still no luck…