Users can't register anymore

Don't know what more to tell you. I've provided the only reasons for the registration save to output that error. It has no further access checks beyond those. My guess is maybe something you've installed is breaking the guest user object or is acting on one of CBs triggers and altering the behavior. Recall what was last changed and revert the change. If you can't recall what was last changed then begin disabling 3rd party extensions and non-core CB plugins 1 by 1 until issue resolves.

Thanks for your help. It is strange that it is not working. I'll dig deeper.

Am I correct that the check is done in the file comprofiler.php?

Thanks for your help. It is strange that it is not working. I'll dig deeper.

You may have enabled forced HTTPS within your CB Login Module to only allow secure login and registrations. Edit your CB Login Module(s) then under Login and Logout Options set "Encrypt Login Form" to "No". If you want to use this feature be sure Joomla isn't already set to force HTTPS globally (if it is you don't need this feature) and that live_site in configuration.php is set to '' (empty).

Am I correct that the check is done in the file comprofiler.php?

Yes, that is the check.

Yes, that solved the problem.

I was doing some testing myself and figured out it had to do with spoofing checks in checkCBPostIsHTTPS function. Still this is quite strange, since a) my site is using https/ssl and b) my testsite has the same setting (encryption turned on) and does work.

Only problem now is that after registration there is an error message: User test@mysite.nl has not been confirmed, which wasn't there before.

The error message seems to be related to AcyMailing. Acymailing apparently tries to add the user to a list, but warms that the e-mail address was not confirmed yet.