Try turning off Enhanced Tracking Protection for Yahoo and see whether that makes any difference. While you are on the page, click the "shield" button to the left of the lock icon at the left end of the address bar, and click the slider switch to turn off tracking protection on the site. Repeat for the login page and the mail page.

Any difference?

Actually your answer did "not" solve the problem. Same results. BUT earlier today a Yahoo support person said to answer "No" on phone, this would reset the Yahoo cookie. I had no better luck with them as with you. BUT together you two fixed the problem. After trying yours and having no success I did the "No" and redid the login and it worked. Doesn't make any sense, third year for this PC, sits here from March to December (all electricity turned off) and never a problem until this time. Usually never even had to re-login.

Thanks much.

What was it you had to answer "No" to?

When when logging into Yahoo you need to (of course) enter username and password. Then Yahoo requires you to go to your phone, load Yahoo there and a message will pop-up informing you someone is trying to login to your Yahoo account. They then ask "Is this you?" You can then select "Yes" or "No." It was me so I selected "Yes" but every time I did it just continued to go back to the initial login page. Yahoo support said to answer "No" instead then login again but it didn't help. After trying what you suggested it didn't help either when I selected "Yes." However, with your suggestion in play, I did it again but selected "No" this time, went back to login and when the verification request again appeared on my phone I selected "Yes" and it worked.

Thank you for the details. It sure sounds like something is broken, but I have no idea what it is.

I have no idea either, but thanks to you and the Yahoo support it's working now and I guess that's what matters most.