Black screen when connecting second display

MacPro running 14.4.1

Processor: 3.2 GHz 16-Core Intel Xeon W

GFX: AMD Radeon Pro Vega II 32 GB

My main display is an Apple Pro Display.

I have a second display I'm trying to connect – a BenQ SW series, connected via (supplied) USB-C.

When I switch the second monitor on, my main Pro Display goes black for a moment, then comes back on in a lower resolution. For a few seconds, I can see the BenQ listed in the Displays window on System Settings (though nothing appears on the BenQ screen). Then after a few seconds, everything goes black again. Finally, my main monitor comes back to life in original resolution, and I can no longer see the second display at all, including when I Option-click "Detect Displays".

I've tried restarting, I've tried a PRAM reset. Didn't make any difference.