failed to connect to '/run/xpra/system':

Hi,

I just tried to use xpra for the first time after upgrading to 18.04. Seems to have problems with the systemd structure (tested on different machines):

$ xpra start :7
Warning: cannot use the system proxy for 'start' subcommand,
 failed to connect to '/run/xpra/system':
 [Errno 2] No such file or directory

It does not start. On a server machine the log file says:

++) Using config file: "/etc/xpra/xorg.conf"
(==) Using system config directory "/usr/share/X11/xorg.conf.d"
2018-09-10 15:36:25,040 created unix domain socket: /run/user/1000/xpra/finnland1-107ESC[0m
2018-09-10 15:36:25,040 created unix domain socket: /home/hadmut/.xpra/finnland1-107ESC[0m
2018-09-10 15:36:25,041 created unix domain socket: /run/xpra/finnland1-107ESC[0m
/bin/sh: 1: dbus-launch: not found
dbus-launch failed to start using command 'dbus-launch --close-stderr':

 exit code is 127

On a desktop machine it says:

/usr/lib/xorg/Xorg.wrap: Only console users are allowed to run the X server
2018-09-10 15:38:12,624
2018-09-10 15:38:12,625 Xvfb command has terminated! xpra cannot continue
2018-09-10 15:38:12,625 if the display is already running, try a different one,
2018-09-10 15:38:12,625 or use the --use-display flag
2018-09-10 15:38:12,626
2018-09-10 15:38:12,627 killing xvfb with pid 27213
2018-09-10 15:38:12,629 failed to kill xvfb process with pid 27213:
2018-09-10 15:38:12,629 [Errno 3] No such process

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: xpra 2.1.3+dfsg-1ubuntu1
ProcVersionSignature: Ubuntu 4.15.0-33.36-generic 4.15.18
Uname: Linux 4.15.0-33-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair
ApportVersion: 2.20.9-0ubuntu7.3
Architecture: amd64
CurrentDesktop: LXDE
Date: Mon Sep 10 15:28:34 2018
InstallationDate: Installed on 2018-04-30 (133 days ago)
InstallationMedia: Lubuntu 18.04 LTS "Bionic Beaver" - Release amd64 (20180426)
SourcePackage: xpra
UpgradeStatus: No upgrade log present (probably fresh install)