@SGaist Ok I've put the environment variable you suggested in my Build Environment list. The application spat some output in which everything looks OK until this line:
defaultServiceProvider::requestService(): no service found for - "org.qt-project.qt.camera"
which looks like something that's querying the D-Bus for some services. I've checked every check box when I was installing Qt using the installer but It looks like it didn't install this or something else is needed but it's not documented.