I am, unfortunately. I tried reinstalling the game, but no luck. I'm currently reinstalling the game to a different drive to see if that has an effect.
Gotcha.
Can you try this workaround maybe? You want to remove all files from the following directories. They can be backed up elsewhere on your machine if desired:
C:\Users[username]\AppData\LocalLow\MCC\Temporary\UserContent\Halo3\Map
C:\Users[username]\AppData\LocalLow\MCC\Temporary\UserContent\Halo3\GameType
C:\Users[username]\AppData\LocalLow\MCC\LocalFiles*\Halo3\Map
C:\Users[username]\AppData\LocalLow\MCC\LocalFiles*\Halo3\GameType
Those are the temporary and saved locations for Halo 3 map and game variants respectively. I strongly encourage you to back up your saved (LocalFiles) variants so you don't lose your UGC.
If that doesn't work, do submit a ticket to our Halo Support team so they can track it.