Must be a recent change, tested with r13763.
tcpdump shows no network traffic at all.
No idea why or how, but the problem fixed itself. I had rebuilt older versions (all the way back to 0.17.0 and before), and all were failing to run. Then it started working again and hasn't failed since. A fluke? (see also #1313)
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1311