Follow up from #1332 which uses python avahi. We need to use a pybonjour listener (not updated since 2009.. ouch) or switch to a better library: python zeroconf.
The mdns server works fine on both win32 and osx. On win32 you must have Apple's Bonjour software installed. (the download link is actually called Bonjour Print Services for Windows now...) r15065 makes the error message clearer.
(edit milestone)
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1333