The problem & solution are now clear...

Two years ago, when I upgraded to EAV v3 I could not log in to a certain forum site (Photography dpreview) and to my Lycos web mail account. After several unsuccessful attempts with the support team, I reverted back to v2.7 and all was fine again...

I kept testing every single new NOD32 version to see if the problem had gone, but no joy.

Very recently, I came across a posting in a forum that talked about an incompatibility of ZoneAlarm with another antivirus product (Avast), not allowing login into Yahoo or something.

I have been using ZA too all along...

That raised my suspicions. I installed the latest v4 and, true to form, no login success. Closed ZA and all worked fine.

I have changed to Online Armor Free now and it works fine with NOD32 v4.0.474. How could I have known then ? (!)... The only drawback now is that I have to get to grips with both NOD and OA but I hope the effort will be worthwhile....