I think I might have had the exact same issue, but I managed to sort mine, here is what I did (fairly easy)
Go to your dashboard, then go to your setting, in there go to console setting and storage settings.
When you are in the storage settings find Forza Horizon and open it up, there should be several save files in there and one (or more) files with an yellow warning circle, those you can delete (nothing else to do I am afraid)
You should not loose any game progress as that is a separate file, and if you have not saved any setups in the past you wont lose any either, I am not sure if anything else is in the same file, like photos and such since I have not saved anything other then car upgrades.
Anyways, that fixed my issue, hope this helps others with the same or similar issues!