Could you capture a Fiddler or WireShark trace of the ProGet response? There should be an error message associated with the 400 (either There must be exactly one package., which would indicate a proxy issue or Package validation failed: followed by an error message, although from a quick glance over the source code, I don't see how that could happen for a NuGet package.)
Another possible cause is an IIS module, like WebDav, intercepting the request before it gets to ProGet.