Each time the preferences view is selected, a new ManageSieve connection is established. Toggling between the preferences view and another one quickly exhausts Dovecot's default limit of 10 simultaneous connections allowed for the same user from the same IP address. If the limit has been reached, the message "Service temporarily unavailable" is shown when you try to save changed preferences. This state persists until Dovecot's inactivity timeout is reached and the connections are closed by the server.