So recently tired playing fm6 and I got a message saying load failed unknown error occurred. Microsoft was unable to troubleshoot the problem. So now my only option is to erase all my game data. Is there anyone who knows how to fix this problem without loosing game data. Help!!!
if it happens when u try to start the game(which means u can’t start the game at all), the problem is likely on the game’s content.
try reinstalling the game may solve this.
if u can successfully start the game while it happens when u try to load an existed savegame…
most likely doomed 
There are loads if threads about this in the Support forum. As far as I’m aware no one has got a response from T10 and no one has managed to recover their progress.
A couple of people, including myself, managed to get Unicorn cars reinstated after starting from scratch by asking nicely (though I don’t actually know which of the messages I sent resulted in mine being reinstated or whether someone from T10 read my forum post).
Before uninstalling and reinstalling, you could try, AFTER checking Xbox Support
Hard reset of your Xbox One:
1 – from the Dashboard, out of all games, hold the X button on the front of the console down until it shuts the system down.
2 – Disconnect the external power from the wall plug (or at least disconnect the wall-side connector on the power pack itself).
3 – WAIT until all power drains from the external power pack (no light, either white or dull orange showing on the pack).
4 – Reconnect the power supply to the electrical outlet.
5 – Restart the console in the usual manner, i.e., either with the controller or a short push on the X button on the front of the X1.
6 – WAIT until the console self-checks and fully boots to the Dashboard before attempting anything.
If you don’t disconnect the external power pack and let the power completely drain out, the system is still retaining “old” stuff and not resetting.