Hi @dario-wigger_0347,
The configuration looks okay, and is basically what we have documented. So I think it ought to work...
From here, I would start by monitoring the HTTP traffic between Maven and ProGet. You can use a tool like Fiddler Classic on Windows or Wireshark. What you should see is:
Maven issues a request, and ProGet returns a 401 with WWW-Authenticate: Basic header
Maven reissues the request, sending a Authorization: Basic XXXXXXX
The XXXXX will be Base64 encoded api:your-api-key. If you don't see that, then something else is wrong. I would post it and maybe we can help.
You can also try downloading maven artifacts from an "Incognito" window. Your browser should also prompt you for a username/password, and then you can enter api and your api key.
Thanks,
Alana