The error disappears if I downgrade Vagrant from 2.2.6 to 2.1.5.
But the version of Virtualbox in Ubuntu 20.04, 6.1, is not supported by version 2.1.5 of Vagrant.
I tried to install Virtualbox 5.2. But I have come across a dependency that I cannot fix: the version of the libvpx package. I have installed v6 and Virtualbox 5.2 need v5.