There's a problem with Yahoo's POP server; server does not support RFC 5746. They have to fix that bug.
Meanwhile, you can work changing security POP switching from "SSL/TLS" to "None". It's unsafe, but you'll be able to download your mail
Other possibility is to set POP server to pop.mail.yahoo.co.uk with SSL/TLS
This problem is happening with Thunderbird, Outlook, Eudora, Apple and other mail clients in different countries.
Forums and succesfull experience