Photoshop Elements Editor stuck in perpetual "Sign In Required" popup mode

Hi,

I have a Photoshop Elements Editor license that has been working fine for the past 4 years, but now every time I try and open the app, I get the "Sign In Required" popup. I follow the steps to login by providing my email and password, but Photoshop Elements Editor never opens. It keeps popping up the Sign In Required dialog. 

I know my product license is registered and active, and I also know my email/password are correct because it's what I've always used and what I also use to login to the Adobe.com website.

Why can't I use the product? How do I get past the Sign In Required dialog box?