Seems to have been introduced with the release of the latest Spartan Ops mission.
Hundreds of Halo 4 players are currently experiencing issues trying to connect to the Halo 4 multiplayer servers, 343 Industries has acknowledged.
The Halo Waypoint forum is filled with posts from players who have been unable to connect to the main game servers since November 26, and are instead being sent to the fall-back servers. This is also causing the temporary resetting of in-game ranks and loadouts.
The problem appears to have been instigated by the release of the latest Spartan Ops missions.
"There are a few hundred players who have been unable to connect to our servers since yesterday," explains a post by the Halo Waypoint forum team. "We're investigating now and hope to have an update soon."
Players experiencing problems should share their Gamertag and location on the Halo Waypoint forum.
"Apologies for the inconvenience," Tweeted Halo Waypoint.
Source: Halo Waypoint