This might not be relevant but I recall that back in the world of $work I could never get keys generated with PuTTyGen to connect via SSH/SFTP to the Linux/Unix servers I was supporting. Installing Cygwin and using keys generated with the ssh-keygen that came with with it worked fine. This was on WIndows XP and Windows 7 laptops/desktops. I've been retired for a few years now so this info may be out of date.