DL-195 + Big Sur = black screen
 

Running Big Sur and a pair of DL-195 that work fine in Mojave to display.
Installed Big Sur (11.2.3) on a new partition to test things out, to discover that regardless of version of the display link driver both displays are blank, and nothing shows up for them in display preferences, but the driver claims there is a monitor detected.

I've tried with one display as well, various reboots, with and without the login extension, added and removed the recording privileges from system preferences.
The display backlight comes on as if it was expecting to get a signal but there is no image.

What kind of diagnostic information is needed here?