After upgrading my ubuntu/focal64 box I got greeted by this wonderful message:
Vagrant was unable to mount VirtualBox shared folders. This is usually because the filesystem "vboxsf" is not available. This filesystem is made available via the VirtualBox Guest Additions and kernel module. Please verify that these guest additions are properly installed in the guest. This is not a bug in Vagrant and is usually caused by a faulty Vagrant box. For context, the command attempted was: mount -t vboxsf -o uid=1000,gid=1000 vagrant /vagrant The error output from the command was: : Invalid argument
The solution was rather simple: A
sudo apt-get install -y virtualbox-guest-dkms inside the VirtualBox guest followed by a
vagrant reload at the host.
In case it matters, the VirtualBox host machine was an Ubuntu 21.04.