Emacs cannot write file in a Virtual Box shared folder

[Bernardo <bernardo.bacic@pobox.com>]

Greetings,

Running GNU Emacs in Debian (9.11) VM guest under VirtualBox host (6.0.14) running on Windows 10 box.
(hope all this makes sense)

Am having problems editing files located in a Virtual Box shared
directory. The directory is owned by root but members of 'vboxsf'
group have read/write/execute rights.  Emacs user is a member of
'vboxsf' group.

 $ groups
  some_user cdrom sudo audio <snip> bluetooth vboxsf

The (old) version of Emacs that comes with Debian reads/writes files ok;
  GNU Emacs 24.5.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.11) of 2017-09-12 on hullmann, modified by Debian

The version of Emacs that's built from Git repos cannot write
file. Also by default it opens the file in read-only mode.
  GNU Emacs 27.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.11) of 2019-10-21

Debugger entered--Lisp error: (file-error "Unlocking file" "Operation not permitted" "/media/sf_Home/zz_stuff")
  write-region(nil nil "/media/sf_Home/zz_stuff" nil t "/media/sf_Home/zz_stuff")
  basic-save-buffer-2()
  basic-save-buffer-1()
  basic-save-buffer(t)
  save-buffer(1)
  funcall-interactively(save-buffer 1)
  call-interactively(save-buffer nil nil)
  command-execute(save-buffer)

Other editors (GNU nano) within the VM read/write the same files correctly.
Sounds like a bug?

--
cheers