admin no longer has access to shared folder

Solution 1:

It sounds like your user is no longer a member of the vboxsf group - you can check by using the id command in a terminal or getent group vboxsf

If that's the case, you can add the user back with

sudo gpasswd --add <username> vboxsf

or the equivalent adduser or usermod command

You will probably need to log off and back on for the change to take effect