I'm not sure if Microsoft just happened to push a fix the minute I tried this but I was unable to play any 360 games that required a connections to Xbox live (B02 Zombies for instance)
I bought my series X yesterday and tried everything including changing port forwarding, turning auto renewal for Game Pass on and off, changing my password to something shorter, uninstalling the game, clearing 360 storage, clearing the reserved space, clearing local game save data.
However I tried again today (I had a day off and really wanted to play MotD) and it still didn't work so I did it again in this order:
1:Uninstall the game.
2: Clear Save Data (for the game specifically) in "Manage add-ons and service"
3: Clear Local 360 Storage (in Manage Storage Devices)
4: Clear Local Saved Games (in Manage Storage Devices)
5: Reinstall the game and DLC
For whatever reason that order of events worked and now I'm able to connect to XBL on the 360 games.
Can anyone else verify the issue still exists and that this solves the problem?