Thank you, @longjohn119 for pointing this out
It works now.
I think the problem was caused by a missing Windows update, a 'functional' update (Funktionsupdate in German).
It is a little bit confusing, because Windows claims that it is up to date but also provides the option to install an additional, non-required update. Which seems to be pretty essential (atleast for ACValhalla)...
Who would have thought...

@longjohn119 Thank you for the advice but sadly it didn't help. In my case, there is not even a folder '13504' in the savegames folder (I wasn't able to play the game yet)

@lecterfan That is the exact description of the problem I have. The games chrashes instantly at the loading screen. I tried everything that is suggested in several guides, couldn't fix it. No reply from the ubisoft support yet. I can't help feeling that the problem has something to do with the ubiconnect SW or the interaction between ubiconnect and ACValhalla.