i found no errors in the production log though, the web ui says it´s saved.
its a 2 day old gitlab 8.2.2 omnibus installation,
last change was ssl activation with a wildcard certificate.
update: exactly the same issue here : GitLab 8.2.1 - cannot disable sing-up and sing-in on interface
update: as mentioned below a gitlab restart does show the new values, the misbehaviour is still in place though as changes are not applied until restart.