I second this. I get a 504 when attempting to register as a new user with an email invite. Using the default config for nginx on aws, no ssh, no domain name. And as @Dem-Ka mentioned, the problem goes away as soon as I change the invite option to any other than Admin Invite Only.
Pretty bothersome, if you ask me, specially if I don't want to queue people for the invite or if I want only to process registries from users I invite.