xpra: unable to start Xserver via ssh

attempting to run a command over ssh fails with xpra in 18.04:

On the remote host:

  $ xpra start ssh:${REMOTE_HOST}:100 --start-child='virt-viewer'
  [...]
  Warning: cannot use the system proxy for 'start' subcommand,
   failed to connect to '/run/xpra/system':
   [Errno 2] No such file or directory
  Entering daemon mode; any further errors will be reported to:
    /run/user/1000/xpra/:100.log
  InitException: failed to identify the new server display!
  xpra initialization error:
   failed to identify the new server display!
  2018-06-19 19:25:57,975 Error: failed to receive anything, not an xpra server?
  2018-06-19 19:25:57,976 could also be the wrong protocol, username, password or port
  2018-06-19 19:25:57,976 Connection lost
  2018-06-19 19:25:57,977 Error: printing disabled:
  2018-06-19 19:25:57,977 No module named cups

/run/user/1000/xpra/:100.log on the remote 18.04 host contains

  /usr/lib/xorg/Xorg.wrap: Only console users are allowed to run the X server
  2018-06-19 19:25:44,199
  2018-06-19 19:25:44,199 Xvfb command has terminated! xpra cannot continue
  2018-06-19 19:25:44,199 if the display is already running, try a different one,
  2018-06-19 19:25:44,199 or use the --use-display flag
  2018-06-19 19:25:44,200
  2018-06-19 19:25:44,200 killing xvfb with pid 27269
  2018-06-19 19:25:44,200 failed to kill xvfb process with pid 27269:
  2018-06-19 19:25:44,200 [Errno 3] No such process

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xpra 2.1.3+dfsg-1ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-20.21-generic 4.15.17
Uname: Linux 4.15.0-20-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.1
Architecture: amd64
Date: Tue Jun 19 19:29:02 2018
InstallationDate: Installed on 2016-12-21 (545 days ago)
InstallationMedia: Ubuntu 16.04.1 LTS "Xenial Xerus" - Release amd64 (20160719)
ProcEnviron:
 TERM=screen
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: xpra
UpgradeStatus: Upgraded to bionic on 2018-05-11 (39 days ago)