Thanks a lot.
I searched for the files and the whole "klang2" folder in "Local" wasn't there at all (so my old save is sadly gone). After playing the intro to create a save it shows up with the save file and everything else.
And just for testing purposes I pressed the "start" button in the itch app again and as result the whole "klang2" folder was gone again.
So it is actually because of the itch app.
Very unfortunate but its always a relief finding the problem. And maybe it'll help someone to avoid the same mistake (if this happens reproducable).
Thanks again for the help, it was a pleasure playing the game, your support is awesome and have a good day :)
Greetings