Comment 4 for bug 1565985

More Info:
I discovered that if I run "sudo modprobe vboxsf", I a different error after running a command like "sudo mount -t vboxsf sfname /sfpath". The error under dmesg is:
sf_read_super_aux err=-22
This error appears to be caused by /sbin/mount.vboxsf not being installed.
(I say that because I reproduced the error message by installing the guest additions from .iso, mounting the folder successfully, unmounting it, and then renaming /sbin/mount.vboxsf.)