synaptic does not show window

[Wolf Rogner]

Tried to start synaptic from Gnome Activities.
Synaptic asks for the password, then it terminates.

From the console I get:
$ sudo synaptic
No protocol specified
Unable to init server: Could not connect: Connection refused

(synaptic:4280): Gtk-WARNING **: cannot open display: :0

I compared the settings (specifically policykit) with those of a similar machine. They are identical.
On the other machine, synaptic starts fine, on this one it does not.

Tried different GDK and GLUTTER backend settings -> no success.

There are other applications that refuse to start with similar error messages (e.g. gparted) and that are essential.

How can I collect more information about this issue?

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: synaptic 0.84.2
ProcVersionSignature: Ubuntu 4.13.0-11.12-generic 4.13.1
Uname: Linux 4.13.0-11-generic x86_64
ApportVersion: 2.20.7-0ubuntu1
Architecture: amd64
CurrentDesktop: ubuntu:GNOME
Date: Fri Sep 22 23:59:26 2017
InstallationDate: Installed on 2017-09-05 (17 days ago)
InstallationMedia: Ubuntu 17.10 "Artful Aardvark" - Alpha amd64 (20170902)
SourcePackage: synaptic
UpgradeStatus: No upgrade log present (probably fresh install)

[Wolf Rogner (war-rsb)]

Not a Synaptic issue