Hi @jan_ko ,
Thanks for sharing this; the .nuspec file looks okay to me, so it doesn't seem like a problem with the package.
After building the agent pushed these nupkg's to the Azure DevOps Artifacts. This feed is connected to a ProGet connector.
Well, I'm thinking the issue is in the Azure DevOps Artifacts API. We found many bugs in their feeds already (i.e. not implementing the API as specified). It happens to work in Visual Studio, or Visual Studio already worked around the bug.
We can spot it very easily with a debugger; if you can share to use a feed url and credentials (I think it's just n access token?), then we can connect to the feed and see what's causing that error. You can send us the details via the ticket, so then it's not public.
Cheers,
Alana