Issue fixed by myself.
For the benefit of who might experience the same issue and for CherylO-Xerox to update your internal knowledge base to provide a better support to other users: the issue was due to the fact that, in the case of putting the MFP in a network, the TWAIN driver connects to the MFP with the T4NET protocol on TCP/UDP port 9400. That protocol was disabled in my MFP (I do not know why), that's why the TWAIN driver was not able to perform any scanning via network.
As soon as I enabled the T4NET protocol in the printer through the Xerox CentreWare Web interface (menu option "Security --> System Security --> Feature Management"), the "scan-to-network-PC" functionality worked like a sharm.
I suggest XEROX to update the MFP manuals, at least in the troubleshooting section and in the SCAN-To-Network-PC section to include the prerequisite of having such protocol enabled in the MFP.
Regards
Mauro Del Giudice