[winswitch] xpra display problem

[Anthony Stone]

I'm running xpra 2.2.4 under Ubuntu 16.04 on both client and server.
When I enter on the client

xpra start ssh/whirligig --start=gnome-terminal

to connect via an ssh tunnel (already set up) to the server on
whirligig, I get the error

Error: displayfd failed
did not provide a display number using displayfd
xpra initialization error:
failed to identify the new server display!

If instead I use

xpra start ssh/whirligig/9 --start=gnome-terminal

I get a session running on display 9, although it seems a bit flaky --
the display background colour keeps changing.

The server system software is newly installed, so maybe some necessary
package is missing. I had no trouble with a previous server setup under
Ubuntu 14.04.

Any suggestions gratefully received.

Anthony