qt platform

Kubuntu 19.10
For a few months now I notice that clicking on the 4kvideodownloader icon doesn't do anything. CL works ; the interface appears and works just fine. BUT when I close it the CL box gives me a pile of aborted core stuff. I have purged the application before trying the latest versions but the last three (four, perhaps) all give me the same result : GUI doesn't work CL does. The problem may be here :

COMMAND LINE : 4kvideodownloader
12:16:51 +0100 (0x00007fb39a013440) [info] [app_Gui] [Translator] Using locale en_GB
12:16:51 +0100 (0x00007fb39a013440) [info] [SoftwareUpdate] [Manager] appId=4kvideodownloader, appName=4K Video Downloader, appVersion=4.10.1.3240, appCast=https://www.4kdownload.com/static/main/app/appcast/videodownloader.xml
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[openmedia][info][20200101_121652][7fb39a00c700]User external ip info: {"ip":"109.219.59.74"}
qt.qpa.plugin: Could not find the Qt platform plugin "xcb" in ""
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

ON CLOSING THE APP
Aborted (core dumped)
12:16:58 +0100 (0x00007fb39a013440) [warning] QMainWindow::saveState(): 'objectName' not set for QToolBar 0x559081d7cae0 '' (:0, )
12:16:59 +0100 (0x00007fb39a013440) [warning] QObject::disconnect: No such signal QObject::changed() (:0, )
[2020-01-01 12:16:59.295669] [0x00007fb39a013440] [warning] QObject::disconnect: Unexpected null parameter (:0, )
IO thread: 85 handlers were executed