Using NAPS2 on a NEW Windows 10 Home x64 PC. Worked fine on my OLD Windows 10 Home x64 box.
Tried everything under the sun so far but no dice. Printing works fine.
Xerox doesn't support or distribute NAPS2. If I had to guess, the difference is likely the Windows 10 build version (1511 vs 1607)
So check your driver version for the 6505 and assuming they match both PC's, go to NAPS2 for support.
If they don't match, then reinstall the right one on the new PC.