OK. As per this thread, setting:
export QT_DEBUG_PLUGINS=1
revealed missing library libxcb-xinerama0. Installing it fixed the problem.
maga@magalab-ubuntu:~/Downloads/Slicer-4.11.20200930-linux-amd64$ export QT_DEBUG_PLUGINS=1
maga@magalab-ubuntu:~/Downloads/Slicer-4.11.20200930-linux-amd64$ ./Slicer
QFactoryLoader::QFactoryLoader() ignoring "com.nokia.qt.QGuiPlatformPluginInterface" since plugins are disabled in static builds
QFactoryLoader::QFactoryLoader() ignoring "com.trolltech.Qt.QStyleFactoryInterface" since plugins are disabled in static builds
QFactoryLoader::QFactoryLoader() ignoring "com.trolltech.Qt.QInputContextFactoryInterface" since plugins are disabled in static builds
QFactoryLoader::QFactoryLoader() ignoring "com.trolltech.Qt.QImageIOHandlerFactoryInterface" since plugins are disabled in static builds
QFactoryLoader::QFactoryLoader() checking directory path "/home/maga/Downloads/Slicer-4.11.20200930-linux-amd64/lib/QtPlugins/platforms" ...
QFactoryLoader::QFactoryLoader() looking at "/home/maga/Downloads/Slicer-4.11.20200930-linux-amd64/lib/QtPlugins/platforms/libqxcb.so"
Found metadata in lib /home/maga/Downloads/Slicer-4.11.20200930-linux-amd64/lib/QtPlugins/platforms/libqxcb.so, metadata=
{
"IID": "org.qt-project.Qt.QPA.QPlatformIntegrationFactoryInterface.5.3",
"MetaData": {
"Keys": [
"xcb"
]
},
"archreq": 0,
"className": "QXcbIntegrationPlugin",
"debug": false,
"version": 331520
}
Got keys from plugin meta data ("xcb")
QFactoryLoader::QFactoryLoader() checking directory path "/home/maga/Downloads/Slicer-4.11.20200930-linux-amd64/bin/platforms" ...
Cannot load library /home/maga/Downloads/Slicer-4.11.20200930-linux-amd64/lib/QtPlugins/platforms/libqxcb.so: (libxcb-xinerama.so.0: cannot open shared object file: No such file or directory)
QLibraryPrivate::loadPlugin failed on "/home/maga/Downloads/Slicer-4.11.20200930-linux-amd64/lib/QtPlugins/platforms/libqxcb.so" : "Cannot load library /home/maga/Downloads/Slicer-4.11.20200930-linux-amd64/lib/QtPlugins/platforms/libqxcb.so: (libxcb-xinerama.so.0: cannot open shared object file: No such file or directory)"
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.
Available platform plugins are: xcb.
error: [/home/maga/Downloads/Slicer-4.11.20200930-linux-amd64/bin/SlicerApp-real] exit abnormally - Report the problem.