[This article is obsolete, see the update here.]
Earlier, I wrote that the current DisplayLink release for Ubuntu was finally compatible with Ubuntu 21.04 (“Hirsute Hippo”). I’ve since learned that I wasn’t entirely correct.
It turns out that if you have a DisplayLink hub plugged into your Ubuntu 21.04 box when you boot it up, it works fine, but if you unplug the hub and plug it back in, it doesn’t recover, and as far as I can tell there’s no way to get it working again short of rebooting.
To fix this you have to revert to the 5.10 kernel that was released while Ubuntu 21.04 was in its beta-testing phase, before 21.04 switched to the 5.11 kernel.
Here’s a script to do that. Download this script, make it executable, run it as root, reboot, and then uninstall and reinstall the DisplayLink (to uninstall, run the installer with the argument “uninstall”).