Thanks Dave,
but I want to fix the error under Windows Update....I understand that downloading the ISO with the Media Creation Tool fixes the issue....but it's a workaround I don't want to try at least for now.
For the antivirus issue, as far as I know is acknowledged using Avast antivirus (for which pcs MS blocked the distribution starting from today while deeply inspecting the issue with Avast). I've got no issues on one of my home pc upgrading it to April 2018 update with Bitdefender installed.