Please don’t take this the wrong way, but it’s been 2 weeks of false claims and going in circles.
It doesn’t seem to be an OS thing…qtcreator is installed correctly and works (though I don’t use it, I’ve just been replicating the issue). From what I’ve read it could be misconfiguration and/or a missing component, but either way it just doesn’t work with that plugin. Apparently even the Qt people say not to use said plugin, you should probably listen to them.
Re-installing qtcreator or the OS is not going to help…you have most definitely proven that. Feel free to keep searching for a solution yourself, just don’t waste too much time on it.
I think it’s time to accept that you will have to write QML, or figure out another way to do what you want.