It doesn't appear to be as widespread as the matchmaking issues that plagued Halo: The Master Chief Collection, but a serious problem has reared its head in Halo 5: Guardians. Some players are reporting their multiplayer progress - their unlocks, commendations, rank, etc. - has been totally wiped, resetting their Spartan to level 1. In addition, these players found themselves unable to connect to multiplayer servers, or booted from matches shortly after they began.
Developer 343 Industries says it is aware of the issue and is working on a fix. It's unclear if this instance is related to similar problems that occurred on launch day - though those seemed to clear up within a few hours or after a few hard console resets for most users, and these new problems do not seem so easily taken care of.
