I have the same Problem, but want to clarify:
The problem (at least for me) doesn’t happen when both participants are Nextcloud-Users. That works as far as i tested.
It happens when you create a public Room and join as guest with the roomlink (https://FQDN/index.php/apps/spreed?roomId=X)
As creator of the room it shows “myself, 1 Guest” on the left side, as guest it shows Waiting for roomcreator to join the call. That shows to me that as a guest it connects and at least can read who created the room.
If the creator leaves the room, it doesn’t change anything on the Guest’s side though.
After install of the app, the file /etc/spreed/webrtc.conf is created with “;authorizeRoomJoin = false”. Changing it to “authorizeRoomJoin = true” and restarting webrtc doesn’t change anything.
Someone got an idea why this happens?
I found this Bugreport: https://github.com/nextcloud/spreed/issues/200
It seems like Firefox sometimes has Problems getting the Webcam, and if the Webcam is not working (or not allowed) then it doesn’t work. They are already working on that problem.
Switching to Chrome or activating the Webcam worked for me.