I've run "xhosts +" and I still can't connect to xpra X server from other hosts. Specifically, I'm launching an application on an LSF farm (hence the other hosts). I don't have the same problem when launching the LSF job from within a VNC session.
I'm not sure what else to try other than "xhosts +".
Figured it out.
By default Xorg is started with "-nolisten tcp".
why "-nolisten tcp"
Because it can be slow and is generally insecure, so it shouldn't be used. Those who really know they want to use it (warts and all) can turn it back on.
this ticket has been moved to: https://github.com/Xpra-org/xpra/issues/1641