?
The error you shared was: An error occurred processing a PUT request to {url}: Invalid user name or password. A PUT request is what the NuGet client does to push packages to a feed.
You'd want to look for a message related to GET or other requests. But, if you're seeing an unknown, then the problem is likely authentication related, but you'd have to look in the error log, it could be a bad url too...
You can always try run fiddler, and attach ProGet to the fiddler proxy server to see precicely the requests that proget is making as well.