OK, so a number of tries later I figured out the issue. When clicking to download an older version of SDK Manager, you are redirected to an “Access Denied Page”. On that page, it tells you you need to be a member of the “Developer program”.
I thought I had already signed up for that by creating the Nvidia account, which then allows you do download the latest version of the SDK Manager software. Apparently not! Clicking on the link about the developer program leads you to a page that shows more about it, and on that page is a button that allows you to request access to the developer program. Clicking on it, you immediately get a message stating that you are approved. Downloads of older versions of the SDK manager now work, but you won’t need them because the login on the newest SDK manager now works, as well.
This was not at all intuitive and it took me hours to figure out, so I am posting here for others that come across this in the future.
Nvidia, it would be much better if the “Got empty programs response from NVIDIA developer server” could let you know that you need to take action to join the developer program!