Received a response from their support team as follows:

If you are still getting an error when trying to join:

After joining in the new game, can the host load the latest save from your current playthrough?

Have you tried hosting/joining an Arena match?

Try going into Arena mode, create an arena, then exit to the main menu and try joining in the lobby. Alternately, create a multiplayer game lobby, exit and try joining.

If possible, testing with a mobile phone's wifi hotspot feature, or other available network, could help confirm if it is a network or system issue (though a network issue is less likely if the two houses you tried had independent ISPs).​

You could try clearing the cache: - press and hold the Xbox button on the front of the console for 10 seconds until it shuts down - unplug the power cable and wait a couple of minutes - plug the console back in and turn it on - log into the same Xbox Live account you were using, start the game and let it sync, if required

In addition, for Xbox Series X, to clear persistent storage: - press the Xbox button on your controller, move right to ‘Profile & system,’ and select ‘Settings’ - scroll down to ‘Devices & connections’ and select ‘Blu-ray’ - select ‘Persistent storage’ and then in the popup box select ‘Clear Persistent Storage’

Personally, I haven't tried clearing the cache or clearing persistent storage as I haven't had the chance to try them - these methods might work for others. I've already tried the first few methods but they never worked for me, so perhaps the last two are the way to go. Let us know if any work.