Yahoo the servers seems to ran into difficulties, many are having the same problem. There are many issues at this time. They hope to have it fixed soon.
In the meantime, you can click one of these links to access your mail, it’s basic, no frills, but it works. http://us.m.yahoo.com/p/mail or: http://uk.m.yahoo.com/p/mail or: http://id.m.yahoo.com/p/mail
This work for me everytime I encounter this problem.
Hope these will work for you too. Good luck!.
After your successful login… you can click your Yahoo icon or the likes to go to the newest version of it again.