There indeed seems to be an issue with Add-ons and GM mode, and more specifically with loading into downloaded areas, especially big ones.
How I solve this is by having a "lobby", a small standard empty room and use that as the login room, either by going there at the end of a session, or by loading up the save in between sessions, do whatever I need to do in preparation of the next session and then make sure the lobby is the active scene when saving so players will login to that scene when we play again.
