I am having the same problem. It does not open with the "new" button either. I cannot compose, much less send, an email, nor can I reply to an incoming email. I have tried in "classic" and the newer version of Yahoo. It seems to have happened around the time Yahoo updated its classic version. Has anyone else had this problem and, if so, solved it?
Try 'new' option beside inbox
To compose and e-mail, you click on the 'new' button...its in the box thats on the left hand side of your comp. Screen