After a peep at the source I theorised that using logRequests=False when constructing the base class, SimpleXMLRPCServer. would see me right, and so it did.

When logRequests is True control ultimately passes to BaseHTTPRequestHandler.log_message which is a write to sys.stderr, meaning the code is unsuitable for running under pythonw.

This might be a special sort of https://bugs.python.org/issue38533 -- special because the code that offends pythonw is in the library.

Anyhow, now I know what the problem is I can work round it, and I'll leave it to you what to do with the ticket.