It’s HostKey + FnKey. By default HostKey is Right (not Left) Control.
Unknown splash screen logon - #5 by Patrick
If that doesn’t work, set up a Shared Folder as documented here:
VirtualBox Guest Additions and Shared Folders
Copy out your Keepass database and any important data. Import a new Workstation. Once it’s running properly, smash the old one to bits (or keep it as backup).