I’ve cleaned my browser cache several times during this debacle.
The problem APPEARS to be exclusively limited to the Inbox Web page - as I can [usually} open “Recent Unread E-Mail” from the main Yahoo E-Mail Web page.
Also, I can right-click and copy the link shortcut of a given e-mail on the Inbox Web page, paste it into the URL box at the top of the browser, enter, and then get to the e-mail.