Not sure if the developers read this forum but if they do they must get something together towards getting a fix released for Error Code 0x803F8001 on launch.
Note - I am aware that it can be temporarily resolved by downloading a free app from the Microsoft Store which seems to reinitialise the accounts.
Unforutnately this is completely unacceptable & is nothing more than a messy workaround, I’m not going to keep going out of my way to download apps from the Store just to get a game working which should work every time without fail. Imagine if for example every time you wanted to run a Steam game, you had to go back and download another one just to get your initial game working, there would be many complaints and rightly so.
I have emailed forzafb@microsoft.com regarding this issue so maybe someone from there will reply.
This is an issue which I also experienced with Forza Horizon 3 in the past. That seems to be much more reliable and has only ever happened once so imagine my dismay when the same thing happens with Forza 7 not only once, but multiple times and I’ve only owned the game for a few days so far.
Please sort this out as a matter of urgency for your paying customers. Not to mention that the Forza games on PC are above & beyond the price of many other recent games so we should expect better service than this.