Problems with starting the game and synchronizing my data (stuck at 0%)

When I start Forza Horizon 4 and press enter to start the game, a smaller window shows up and says “Daten werden synchronisiert” (german language because german game).
Underneath that is a progress bar that stays at 0 %.
I have waited for about an hour but it still stays at 0 %.
I am using the same account in the xBox app and in the windows app store.

I signed out and signed in again.

I have reinstalled my game.

Opened ports for the game in my router.

Spoke with the customer support of xBox.

When I start the game in offline mode and then reconnect to the internet, I can’t play the online mode, because the game isn’t able to connect to the server.
(Thats why this isn’t a solution too)

I hope that someone can help me.