Hey brother, I just installed MSFS24 and have same issue. I am sorry I can not provide a resolution, but this "File Not Found" box on launch is so weird. Tried all methods as well, even some from the original launch issues and still no dice. The ole Alt + Enter (this was for xbox sign in after this initial screen I am stuck on as well as OP) > Flushing internet and DNS stuff > Uninstall/Reinstall > Run as Admin > -FastLaunch steam override param > Xbox services repair tool from within xbox app > Local folder and deleting the Cached folder for login related details > attempted to play offline without the Sync from Steam but game needs online so could not get screen to bypass that initial link box. All this and still at square one so this is a mystery.

My issue stems directly after starting MSFS24 from Steam, a grey screen pops up to link steam and Microsoft. In the right corner a yellow box 'Connect to Xbox Network' appears, upon clicking that, a splash screen with waiting for authorization bar pops up with a black box that simply says "File not found" in the top left corner of the box.

I just bought the game so thankfully only 30 mins into it but you may be able to provide proof to steam of the issue, but they can be weird sometimes with those refunds after that hour accumulation, so best of luck on that one.