Synaptic opens a blank window

[Dr. Heiko Muenkel]

After clicking in a box in front of a package a blank window occurs instead of a popup menu with entries like "mark for installation". Therefore it is not possible to install a package.

lsb_release -rd
Description: Ubuntu Focal Fossa (development branch)
Release: 20.04

apt-cache policy synaptic
synaptic:
  Installiert: 0.84.6ubuntu5
  Installationskandidat: 0.84.6ubuntu5
  Versionstabelle:
 *** 0.84.6ubuntu5 500
        500 http://de.archive.ubuntu.com/ubuntu focal/universe amd64 Packages
        100 /var/lib/dpkg/status

ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: synaptic 0.84.6ubuntu5
ProcVersionSignature: Ubuntu 5.4.0-21.25-generic 5.4.27
Uname: Linux 5.4.0-21-generic x86_64
ApportVersion: 2.20.11-0ubuntu24
Architecture: amd64
CurrentDesktop: KDE
Date: Tue Apr 7 23:18:17 2020
InstallationDate: Installed on 2020-04-06 (1 days ago)
InstallationMedia: Kubuntu 20.04 LTS "Focal Fossa" - Beta amd64 (20200404)
SourcePackage: synaptic
UpgradeStatus: No upgrade log present (probably fresh install)