Your login module login redirect URL and your first login redirect URL are both empty so it attempts to return the user to their previous location. In this case it can't as that's the confirmation page so it'll error. Set the first login redirect URL within CB > Configuration > Registration to index.php and that should resolve it.

However our return redirect behavior should better ignore situations like this and just send to home page. Have added a bug ticket to see why it's allowing login redirect to confirmation page.

forge.joomlapolis.com/issues/6249

The login redirect should simply be index.php do not include a domain or anything else.