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I am one of the developers, which is why I can assure to you that ProGet is not issuing a 503. This is IIS.
However, based on the new information you've provided, it's likely happening because of an abrupt Application Pool termination such as a access violation, stackoverflow, etc. In such a situation, it's not possible for ProGet to log anything; the program (AppPool) immediately crashes. Then IIS issues the 503 error.
However, IIS should be logging these in the Windows Event log; so if you can look into what it is, then we can try to debug it further.
This is an easy scenario for us to set-up, and it's a common error condition, but we cannot reproduce it.