I ran into a problem in the VirtualBox: the added shared host-folders could not be seen in the Windows Explorer in the VM.

Solution: Remove the Guest Additions and Connect is again and run the setup.

Here is the detailed info, found on the internet:
https://forums.virtualbox.org/viewtopic.php?f=7&t=98998
-> imporant is the next info:
Re: Shared folders not showing in explorer on Win10 guest
Postby Makarios » 13. Jul 2020, 22:29

Ok All the other Vm’s seem to be ok: Shared folder working fine.

So I tried something really innovative:!!

I disconneccted then manula mapped connections ot then network shares with the host.

Then I ejected the Guest additions CD : VirtualBox menu: Devices >> Optical drive >> Remove disk from optical drive.

Next I inserted the Guest Additions cd image again by Devices >> Insert Guest additions CD image.

When the popup came on when the CD image auto ran, I ran the guest additions installation again,

And Now all is well!!.

Problem solved.. Just needed to re install the guest additions all along.