After adding the shared folder Websites
to fstab
by adding the line
Websites /var/www vboxsf rw,uid=33,gid=33 0 0
and giving the right permissions, I always got the following error during my virtual Linux Mint boot:
keys:Press S to skip mounting or M for manual recovery
All I had to do was add vboxsf
on a line of its own in /etc/modules
to force loading it early.
Solution found on askubuntu.com