Konqueror does not show video thumbnails

[Skyman]

1) Release: Kubuntu 18.04.2 LTS
2) Version: 4:17.12.3-0ubuntu1
3) Expected behaviour: Konqueror should show thumbnails for video files, if kffmpegthumbnailer and ffmpegthumbnailer are installed.
4) Actual behaviour: Konqueror does not show thumbnails for video files. But dolphin does.

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: konqueror 4:17.12.3-0ubuntu1
ProcVersionSignature: Ubuntu 4.18.0-20.21~18.04.1-generic 4.18.20
Uname: Linux 4.18.0-20-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.20.9-0ubuntu7.6
Architecture: amd64
CurrentDesktop:

Date: Sat Jun 1 20:24:48 2019
InstallationDate: Installed on 2019-05-30 (2 days ago)
InstallationMedia: Kubuntu 18.04.2 LTS "Bionic Beaver" - Release amd64 (20190210)
ProcEnviron:
 TERM=xterm
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=de_DE.UTF-8
 SHELL=/bin/bash
SourcePackage: konqueror
UpgradeStatus: No upgrade log present (probably fresh install)