If the feed is reporting "healthy" that means that an initial check passed. The 500-status errors will be logged on Admin > Error Logs; without seeing those, it's not really possible for us to advise on what the issue is. Hopefully that will give insight as to what could be failing. You may need to attach a proxy server between ProGet and MyGet (e.g., install Fiddler on the server, then point ProGet to use the Fiddler proxy), in order to capture the traffic.

Hey, This is great news! Thanks for getting this sorted out so quickly! Gary