Just posting here to confirm that the issue was resolved. Still a frustrating experience though...
And the solution was... what? Was it an user error, or bad AV product, or something at GOG's end?
AV false positive, as was suggested (took them some time to reply, but at least the latest virus database upgrade fixed everything). Going to try some other software this summer. See, I've had cases in the past where simply ignoring the warnings caused problems because there was
something wrong, so I tend to be a bit paranoid about this. (Honestly, no AV software is perfect, so you never know .)