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Had this issue one night last week. A friend brought over his xbox and we did a few four mans with local coop on two tvs just to try it out. All was fine till we took a break and came back a while later.

Found a thread in Console Discussion here (2.6k views!) but not for multiple consoles behind a NAT. Did find posts elsewhere that reference when this issue happens for two xboxes behind one NAT router, but none of the ideas there worked.

Figured it probably had to do with the sequence we booted up the units earlier in the day then when the first xbox that was booted up went to sleep and was brought back online it couldnt get back into the game (showing this error).

Maybe it was hosting a component that was being relied on for the connection and unsleeping it brought up a new instance of that component. With the other box still looking for the old instance.

Rebooting it failed. Turning it off, then booting it up failed.

What worked was power off both devices. Then power them both back up. Did that one at a time, but it may or may not have mattered.

Assuming this is repeatable then I guess the motto is dont let the first one booted up go to timeout if you dont want to have to power off both to get going again.