After upgrading to Catalina 10.15.4 (from 10.15.3) my DisplayLink monitor stopped working on my MacMini.
I read stuff here. I checked that the extension was still installed. I checked that it had permission to read the display. I rebooted. I uninstalled the Beta-version driver and reverted to the Release-version driver. I checked everything again. DisplayLink monitor remained blank.
Same monitor, same software on my MacBook: works fine.
Then I followed the directions
here to report an issue. Downloaded the report tool, and ran it. Then I realized that the DisplayLink monitor was still plugged into my MacBook, so I reconnected it to the mini.
It worked.
Thats right — the only thing that worked was running the reporting tool (while the DisplayLink monitor was
not connected). Maybe it kicked the USB bus somehow? Very strange.