You might want to check if all your DNW are hooked to a proper potential. It seems that you have a floating well somewhere in your layout. If your layout lvs says that it is clean but you still have this error, you might have a virtual connect flag turned on during the verification run. A physical connection is required to get rid of this error.
I already fixed it already before I read the replies.
Your right that the NW layer that was covered by the DNW layer must be connected to the same potential...I found out that some of my NW are floating...I just did connect all of them at same potential and run LVS and the DNW and NW Stamp error mult are gone.