Ubuntu VirtualBox client unable to mount shared folders

VirtualBox version: 4.3.10r93012
Ubuntu version: 12.04.4

Error when attempting to mount a shared folder: mount: Protocol error

Issue:
lrwxrwxrwx 1 root root     40 May 29 22:45 /sbin/mount.vboxsf -> /usr/lib/VBoxGuestAdditions/mount.vboxsf

This link was incorrect, even after installing the guest additions and running through the procedure as per https://www.virtualbox.org/manual/ch04.html#idp55330864

Fix:
#cd /usr/lib
# rm mount.vboxsf
# ln -s /opt/VBoxGuestAdditions-4.3.10/lib/VBoxGuestAdditions/mount.vboxsf

After this, manual mount worked fine and the box rebooted cleanly.