This may be a NAT issue like I had. When online check your connection settings to see what type of NAT you have. There are 3 types: strict, moderate & open. Open is the least restrictive of the three. Mine was strict and I could never join in on the league
races until I changed it. Net search "xbox one change nat" and you will be presented with a number of sites offering a solution. If you aren't tech savvy pick one which you find easy to understand as it can seem a very daunting process. Get it right and you
will be punching the air like I was!
I apologise for posting a possible solution to this players problem. I have been recieving these error messages since day 1 and changing my NAT status last weekend fixed it. Up to then I only managed to races leagues once and race multi-player once, not
due to lack of trying.
Online play for most is broken. The November patch has made it worse. Its all over the forza forums. There are a lot of unhappy people. T10 are saying its a Microsoft problem Microsoft say the problem is with the developers T10.
this means we shouldn't expect a resolution anytime soon.