Jep, same here. Windows 10 x64, just downloaded the installer, but it keeps telling me "wrong password".
It is obviously not wrong as I can login to the GOG website without problems.
And yes, I'm using the email address and not my user name in GOG Galaxy =)
Some more info, maybe it helps with finding the issue: (it's a german installation of Windows 10)
Edition Windows 10 Pro
Version 20H2
Installiert am 17.12.2020
Betriebssystembuild 19042.746
Leistung Windows Feature Experience Pack 120.2212.551.0