I ran export QT_DEBUG_PLUGINS=1 but there was no output printed on the terminal.
You should run the command and then the JM GUI in the same terminal, did you do that?
I did. Still nothing.
You forgot to type export before the command. It should be:
export QT_DEBUG_PLUGINS=1
By exporting a variable, you are making it visible to child processes.
What this is supposed to do is print debugging information about the shared library that has failed to load and thus causing the plugin error. It should print output similar to this:
Got keys from plugin meta data ("xcb")
QFactoryLoader::QFactoryLoader() looking at "/home/inso/Qt/5.15.0/gcc_64/plugins/platforms/libqxcb.so.debug"
"The shared library was not found."
not a plugin
QFactoryLoader::QFactoryLoader() checking directory path "/home/inso/build-untitled-Desktop_Qt_5_15_0_GCC_64bit-Debug/platforms" ...
Cannot load library /home/inso/Qt/5.15.0/gcc_64/plugins/platforms/libqxcb.so: (libxcb-icccm.so.4: Kann die Shared-Object-Datei nicht öffnen: Datei oder Verzeichnis nicht gefunden)
QLibraryPrivate::loadPlugin failed on "/home/inso/Qt/5.15.0/gcc_64/plugins/platforms/libqxcb.so" : "Cannot load library /home/inso/Qt/5.15.0/gcc_64/plugins/platforms/libqxcb.so: (libxcb-icccm.so.4: Kann die Shared-Object-Datei nicht öffnen: Datei oder Verzeichnis nicht gefunden)"
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.
The bold parts are the key information we need to find. If we know the library we can install it with apt.
In your case, the problem may or may not be libxcb-icccm, that's why I'm asking for debug output first.