Hello,
I have the exact same issue on my UX480FD
From a new installation, the extension screen was working fine. But after I have installed the Asus drivers, the extension screen is black.
How do you uninstall all the screenpad related drivers exactly?
I'm not even sure the drivers are correctly installed, all we can find on Asus page are zip with CMD files and readme files that are not corrects. The CMD file output is not usable (only 0 and 1, without any explanation, and some "information not found" messages) and we don't even know which device is connected to the driver...
These drivers are the worst packaged drivers I've seen.