Thank you very much!
I looked through your code and some parts were confusing for me (leaving room after a left room triggers) but I will try to figure it out and read all of it again.
After seeing your code with the groups I was curious and just tried to add a System>Wait action and it seems this fixes the whole thing, too!
In the "On signalling left room" part I just added System>Wait 2 Sek (seems 1 works too) and then I execute the new Join Room command and it finally worked. So it seems you just need a small extra pause after the "Left room" trigger before you can join a new room. But this almost seems like a bug for me since the trigger should only run when you completely left the room, right Ashley?