I don't know enough about VBox shared folders to be of any reeal help to you, but perhaps there is some conflict between VBox shared folders and Samba? Perhaps check Samba error logs and see if there is anything of interest there. I'm not sure exactly what it'd be called but I would imagine you will find it in /var/log (that's where all log files should be).

Another attack you could try is to set up a Samba share somewhere else (that works - like you said you've been able to) and put a symlink to the VBox share in that. Not sure if it'd work but could be an easy hack if it does.

The other options are to install Samba on the host system and share direct from that (or you could try sharing that through your fileserver appliance). Or as your other host system is Linux too, you could try using NFS (Linux native network filesharing) and mount the NFS share in your fileserver and re-share that using Samba. Although TBH, re-sharing folders from your fileserver appliance may not work either if the current problem is a limitation of Samba (ie you may hit the same wall?)