Comment 11 for bug 1760371

Apparently, I've had VirtualBox guest utilities installed on my host system which you are generally just supposed to run on the guest. This issue only occurred when the package was updated. After trial and error, I've found that the offending package is virtualbox-guest-x11:amd64 (5.1.34-dfsg-0ubuntu1.16.04.2), none of the other virtualbox-guest packages cause this problem.