I am now 100% sure the problem is on yahoo side and sent this email to them.
I will update with their reply when I get it.
________________________________________________
I am a Mail premium customer, it has been 3 days now that I cannot log on the IMAP server on any device: I have tried Outlook 2010 on a windows 7 computer, OSX Mail app, Iphone, Ipad ...
My credentials are correct, because I CAN log with those same credentials on the POP server and of course the webmail.
Now another proof that the problem is on your side: on all mail clients: Outlook, OSX Mail app, Iphone, Ipad, if I use my wife's yahoo mail credential (she has also premium mail) with all other settings equal for the IMAP sever, (SSL, 993, imap.mail.yahoo.com) I can log in, and as soon as I change back to my user name and password I am rejected. -----> Remember with my credentials I can log on to the pop server and the webmail.
So for sure something is blocked on your side for my account on the IMAP sever,
Please fix it.
Thank you