If you have a eg: (Att) Yahoo account like the below eM Client forum POP user below, they had to create a "OAuth security key on their Att side and put that as their account key which then fixed the login pop problem.
So was just a thought if you have an Att Yahoo account or have an option to put in an OAuth key in your account end.
I did a little research and decided to create the Oauth security key on the ATT side and put that key in the eM Client account password field and viola, works perfectly again.