After our last chat, one other thing occurred to me. Have you ever joined the vboxusers group? I've always joined that because it gives me more options for USB, and I've used the host's USB ports a number of times.
To add my user name to that group, in the terminal I've started an editor (micro for me, but nano or others will work) and then loaded /etc/group into the editor. At or near the bottom, you should find the VBoxUsers group. I added my user name (jerry) to that group and saved the file.
That's the only last thing that might be different between my situation and yours. But if you've already done that, or used a different command to join that group, then I'm out of ideas.
EDIT: I'm hoping that if there's someone on the team who has a better idea, you'll hear from them. This is indeed a really puzzling issue.