Your request is prohibited because it would cause a cycle.
I was trying to log into my Yahoo Mail today and received this message
"Your request is prohibited because it would cause a cycle."
The link I was using was to the Old Yahoo Mail site, so the thought was they've closed the link to force users to start using the New Yahoo!
I changed the old mail link to this and it seems to work. Give it a try and see how you go.
Remember to sign out of Yahoo Mail first as I'm not sure if it will work until you do?
Note that using a specific URL that includes GET data or uses a specific domain like mg5.mail.yahoo to access a specific page can stop working at any time there are changes made to a web page, so it is best to avoid bookmarking such pages and use a button on the main page or the log in page.