Many thanks for your reply!
This is the config on Steamworks
This is the config in the project settings:
The Web part is not configured (and not needed, right?)
Here I upload it (Of course there are all the save functions before this. After saving everything, I do a upload like shown below):
This is how I download it from Steam and then load it (According to the build feedback from Steam, no files get downloaded):
This happens on Main Menu construct, which is the very first thing loaded, before everything else.
Additionally there is of course all the stuff with the SDK that has to be at the right place and so on. But I’m regularerly uploading builds, achievments are working. This should all be fine. Without having added the App-ID within the correct files and stuff, nothing would work.