I am also having this problem, and I have some more details. I think there’s something wrong with the account that I can’t see. The problem happens on 2 different shield TV boxes (on on the same network as the server, the other is remote). But it only happens on the one managed account.
I created a new managed user and that new user can start a “watch together”. I can’t see what would be different about this new user from the old one. I tried to make the user as much like the old one as I could (same access to the same libraries, etc.). I would just switch them over to the new managed account and delete the old one, except that would lose the watch history.
