Once mattermost is launched, if you stop it with rcctl (or any other method I have tried) netstat still shows connections even though the server is stopped. This seems to prevent it from being usable again if you try to start it because it can't bind to the port again. I don't know of a way to clean these orphaned connections. I have only been able connect to mattermost again after a reboot (there must be another way to purge these connections?).
--TimH
