try to login from here for faster and error-free access
http://us.m.yahoo.com/p/mail

and

It seems you are using new yahoo mail, which needs latest browsers.For error free emailing use yahoo mail classic

Use the below link to return to yahoo mail classic and click yes ,when it asks for "opt out"
http://us.mg1.mail.yahoo.com/dc/optout?s

for co.in account,
http://in.mg1.mail.yahoo.com/dc/optout?s