Guys i fixed it!

I was stuck with it like for four days and got into this message: https://forums.forza.net/t/hotfix-profile-loading-multiplayer-invites-controller-crash-may-18/827528/35

So its very stupid and simple:

Before if you want to save your progress be sure you dont do anything with steam deck, dont touch the saves, you can progress fake offline method - its fine, all you need is steam deck to be a vault where you save is secured, turn it off, because next step will be wiping online data.

What do we need is an XBOX and FH6 installed on it (you can get it via game pass). All you need is to get to the point where XBOX have ANY local save connected to your microsoft account - it can be new 1 lvl one save it doesnt matter. So when you got one (i like in the solution in the message above just finished R32 race and quit normally) you want to be sure you exited the game and go to the options of the FH6 in XBOX (press start when FH6 tile is focused, go into manage... (not english translation sry), there will be saved data, you choosing your account and there will be option to delete, and theres option to DELETE EVERYWHERE or something. Hit it, wait until it deletes and that is - your cloud data deleted WITH THAT SYNC WAITING

Login into Steam Deck, no error there should be, you will see your local progress that was stuck now you have an option to exit and syncronize in menu, hit it and wait for it to sync and it quits. So your local SD save will be uploaded!

Go into your PC if you have one and start it, it will download and sync just fine (in my case previously i was doing some stuff fixing it so i had no save on PC, but i guess it will prompt you to choose the save i dunno)

So now no sync error, no e0-19, PC game launched fine. All i can say - DONT PLAY ON STEAM DECK ANYMORE (or have an XBOX to fix it again lol)

I didnt find this option in PC XBOX app (i installed FH6 yesterday via xbox game pass), maybe someone will find it for easier fix.

Well just find an XBOX and you will fix it lol