The culprit was Bitdefender (my antivirus). If you have this problem, go to Bitdefender's Settings / Advanced and set "Scan hosts file" to Off.

I updated my hosts file (eventually!) to enable access to a networked drive. Entry was "192.168.1.7 NAS2 " (without the quotes) and Eureka! it worked.

BUT (and it is a big but), it failed again...

FYI Setting up a brand new laptop with win10 Home preinstalled I couldn't map my NAS drive even though I could log on to it, then I completely uninstalled McAffee and all is working fine now!