I’m playing on my computer threw Xbox game pass and my friend is playing on his Xbox One X. We got the game together and played the for a few hours together. We didn’t play for a few weeks and now we are trying to play again together and it keeps giving us a “Connection Failed” box with the message “Failed to join the multiplayer session, or the session is full. (0x00000001)” it gives both of us this error whether we try to join each other or try to accept invites.
Both of us are online and can join other people’s open multiplayer games just not each other.
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The same problem. We tried to connect Xbox GamePass and PC GamePass, after 2 hours, and as a result, my saves on the account were deleted. 
Me and my friend are having the same issue. We found the game a few days ago, and want to keep playing together, but the second time we tried to join the same time, it wouldn’t work, getting the same error message as the original post. It should be noted that we did try playing through xbox gamepass to pc gamepass and pc gamepass to pc gamepass, and got the issue both times.
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Also, we are able to play regular multiplayer just fine, but are unable to join each other, in co op worlds or full multiplayer worlds.