We had this problem with recently but I haven't seen any notes lately. One thing to check is to make sure you are using your full email address in the server settings username. Without the @yahoo.com or one the other yahoo supported domains (which would need added to domains list) web-mail will not be able to find the correct extension and give this error.
This doesn't fix your problem we need to get the webmail author involved so he can troubleshoot your problem. Eugene
