Have spent 11/2 hours with xbox one support ruling out issues from xbox one hardware and software.
Loaded game played first couple of races at part down load stage .game fully download then wouldn’t play at all just went to a cycle loop while trying to load.
now I have deleted, reloaded game. Exchanged disk 3 times . reset xbox one .reloaded game with Internet connection and without. Still no joy game doing same loop load.
added a new profile of my partner turned off xbox ran game (Google to find this fix ) and it worked on partner profile . But the only way it will work on my profile is to join in split screen races then after a couple of races it kicks me off saying controller disconnect and when i join back i have lost all leveling up of my car.
I tell you in all my years gaming I have never had so much trouble with a game please help
Considering it works with the other profile, it sounds to me like your save game might got corrupted.
Have you tried deleting it and starting a fresh save game?
try an external drive. maybe the intenal is a bit unresponsive
When you say “deleted and reloaded game”, do you mean a complete disc uninstall/reinstallation? If not, use these procedures to remove only the Forza 5 install; then, reinstall.
Hopefully Xbox Support also had the OP recover the gamertag on the system. According to the posts, a secondary account works fine, and to me that points to something being invalid in the main tag. I would make sure that is done first.
Ah, yes. That thing about other Gamertags working properly is important. It sure seems like recovering the Gamertag should work. Give it a try oNEFARIOUSone.
I’m wondering if this might fix the problem I’ve been having with certain tracks either not loading or dashboarding in the middle of the race?
How do you “recover” the gamertag? Will this delete my progress in Forza? I have already done all the troubleshooting steps including resetting game save at start screen. If recovering my gamertag will not delete my progress I would like to try it. I’ve already played too much to delete my progress again so if that’s the case I won’t bother.
Sorted it out went into downloaded game Achievements for forza 5 deleted all files to my profile then deleted profile from xboxone restarted xboxone started forza game reloaded my profile all good only took 3 weeks to sort how about some free car packs to take away the headache lol.
having sick fun on forza now 
Recovery of the gamertag will not remove individual game achievements. They’re stored separately in the Xbox LIVE records.