[Signup] Permission denied also if signup is enabled

The SignUp view raises a PermissionDenied also if the signup is enabled via preferences

added part::backend prio::1 source::customer type::bug labels

assigned to @debdolph

created merge request !911 (merged) to address this issue

mentioned in merge request !911 (merged)

closed with merge request !911 (merged)

mentioned in commit d09fc4ed