Just stopped working (Germany). I reached 1000 hours yesterday and got the achievement. 
Today I am greeted with „Please insert the Microsoft Flight Simulator Game Disc“. That is a new one for me. I had the „Cannot authenticate“ message a few times before. Every time it seems there is something broken in the underlying services and the messages are not really useful, perhaps even misleading.
I will give it a day, see if it fixes itself. Maybe two month ago the startup on XBox stopped working for me. That seems to be connected to cloud syncing and the controller. If I disconnect the controller during startup, before the syncing happens, by removing the battery, the sim actually starts on my XBox. And now these problems on PC. I am not sure MSFS is to blame. It seems like there are issues with the underlying XBox services layer, but it is very annoying.