Offhand, not sure if there's an answer to a similar question here yet, but this is something that's been broken since the release of 20.04 alas. I'm not sure if it's fixed in the release of 20.10 or 21.04, as I haven't tested those or looked into this.

Fortunately, the folks at DisplayLink made available a workaround noted in a pinned forum post.

I would suggest trying the link above, in case the instructions change, but since Stack Overflow prefers the answer include steps to avoid bit rot:

(Edit 1-2022: The link below is no longer available; did not find an alternate source; see the other Answer)

# Download the debian file from https://www.displaylink.com/downloads/file?d=310

sudo dpkg -i xserver-xorg-core_1.20.8-2ubuntu2_amd64.deb

For the juicier details here's the issue at gitlab for xorg/xserver.

But! There's one more step you want after you install that workaround package... you want to avoid installing the package from mainline, so you can do:

sudo apt-mark hold xserver-xorg-core

(you can undo this with apt-mark unhold xserver-xorg-core)

Less useful, but an additional 'workaround' is that this issue (for me) does not happen if there is a non-DisplayLink screen also attached to the machine (e.g. HDMI) at the same time. Had the workaround debian package not been made available, I probably would have ended up adding a HDMI EDID-emulating dongle to my DisplayLink-only system, but fortunately did not have to do this.