Some Yahoo mail users are experiencing problems accessing their email via the web. Their user name and password is correct and they can still access their email using IMAP from a mail client. But when they attempt to access their email using a browser they get caught in the login loop. Even after trying the steps recommended by Yahoo e.g. Login loop: the sign-in box continues to reappear the problem remains.
One additional solution is to change your password to one that does not use special characters. Make it long but only use upper & lower case characters and numbers.
How do you change your password when you’re caught in the Yahoo login loop?
 USE YOUR BROWSER TO LOGIN TO YAHOO
You will know you are logged in because displayed at the top right of the page is “Hi, <Name>”.
 NAVIGATE TO THE PROFILE PAGE
 OPEN ACCOUNT INFO
Hover over the gear icon and click Account Info
 CHANGE YOUR PASSWORD
Sign-In and Security > Change your password