Fragment more info.  First a typo in the description, at the end of the MTR "unexpected connection" should read "unexpected disconnection" of course (no-one expects the unexpected connection ho ho).

Looking at calls to socket functions, Procmon shows the client repeating the call to a server running in pythonw.exe. It seems as though the client is instigating the badness, but...

In a Wireshark capture each of the two sessions the client holds with that 'bad' server is identical, and the difference between each of those and the 'good' session (with the server in python.exe) is that the server under pythonw doesn't return a methodResponse packet, but instead begins tearing down the connection.  Which is what the exception message says, but it's still startling to see it actually happening.

I'll have a squint at the source.