usually host-shared-folder get auto-mounted during boot via vboxsf kernel module.

symptoms:

# manually mounting the by-host-shared-folder "projects"
# as user with uid 1000 and groupid 1000, results in:
mount -t vboxsf -o uid=1000,gid=1000 projects /media/sf_projects
# also try:
mount -t vboxsf -o uid=1000,gid=1000 sf_projects /media/sf_projects
/sbin/mount.vboxsf: mounting failed with the error: Protocol error

dmesg
# reports
sf_read_super_aux err=-71

possible causes:

the folder actually exists, but maybe your host-shared-folder is actually a soft link to an actual folder.

try to change to a different folder if that mounts well.

if that does not help:

you may have copied this machine from another host, and the vbox guest additions version might be too old.

try to update it.

# you can check what version of guest addition is installed
# check what modules are there
lsmod|grep vbox
vboxsf                 36466  0 
vboxguest             189745  5 vboxsf
# check version
modinfo vboxsf
filename:       /lib/modules/3.2.0-4-amd64/misc/vboxsf.ko
version:        5.2.6 r120293
license:        GPL
author:         Oracle Corporation
description:    Oracle VM VirtualBox VFS Module for Host File System Access
srcversion:     D1DF1926BDE4A7635B819EF
depends:        vboxguest
vermagic:       3.2.0-4-amd64 SMP mod_unload modversions 
parm:           follow_symlinks:Let host resolve symlinks rather than showing them (int)

Links:

https://www.virtualbox.org/ticket/928

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