I installed another instance of Zorin 16.3 Core onto a Windows 10 machine using VirtualBox. The host OS is Windows and guest OS is Zorin. It seemed to install properly. However, to enable file sharing between Windows and Zorin, according to the instructions, I'm suppose to go to Devices and select Insert Guest Additions CD Image, but when I do that, nothing happens. Then I selected Upgrade Guest Additions. That errors with a message: "Automatic update of Guest Additions has failed; files were installed, but kernel modules were nor reloaded automatically. Please consider rebooting the guest." I did that, but there was no effect.
This problem seems to have cleared up after I reinstalled Zorin in VirtualBox. I don't know why, but reinstallation seems to work wonders.
I am really glad. This particular issue has been a thorn in my side multiple times in using VirtualBox.
I was struggling with writing a support post to try to help.