"shared folder" does not work

[Byroniac]

Looks there was not a clean uninstallation. Does anything remain? E.g. something like /etc/init.d/virtualbox-guest-utils or whatever. Plus double-check your mount location. Try leaving the mount location blank, then running "mount" (or look at /etc/mtab) and see if it lists it as a mount point. My guess is the uninstallation is not complete. But if not that, then I am not sure at this point. Thank you for being patient.

[Byroniac]

Well, you don't have a clean uninstall for one thing. You might try something like 'apt remove --purge virtualbox\*' and see if it can remove anything else. Until you get a clean install and/or uninstall, I doubt your extensions will ever work.