First, we don’t have “back-ups” and haven’t had those since cheaters ruined the ability to do so back in FM2. Just totally forget a “back-up” because - just like on any Xbox 360 Forza game, you’ll get a corrupt game save message. Your data in a “back-up” doesn’t match that which is the latest true game save so the anti-cheat software kicks in on the servers and invalidates it. Then you’re not allowed to play the game until the save game file is deleted and everything reinstalled.
Have you also had to delete your saved data in order to get past the load screen? have you tried any method of resolving the issue? Looks like a bug which can effect all of us which really needs to be sorted out.
Microsoft Games will tell you that your game is always backed up on their servers. But if you inadvertently save a bad version or something goes awry on their end, you’re screwed. I lost my game when I had affinity on every car brand, I owned every car, had tunes and customization on hundreds of cars, and about 200 million credits. I feel your pain. But, like I said, we’re just plain screwed and there’s nothing we can do about it.