What title did the html in the validation .rip file show?
hint: search help for ".rip"
When you execute the validation, if there is an error that causes validation to fail, a capture of the request and response is stored in the logs (log file tab in workbench) with the extension .rip.
Nobody can clearly explain how to resolve your issue without more details. Basically, the response from yahoo mail (or your app) was not as expected and is most likely due to a bad request - which can mean many things...
A problem is a difference between what is perceived and what is desired, that
we want to reduce (Dewey 1933)
It seems that your Yahoo session has expired. Each you login, Yahoo will open a session for you. After you logout, the session is terminated. So, when you compile, no error. But validating is trying to access the session when you were recording scripts. The possible solution is to parameterize the Session ID to a variable to catch and replace the session Id where applied.