Similar problem - I couldn't work out why earlier I was unable to log into a client's webmail, even though we were sure we had the right username and password.
Just came back to it and realised that clicking on the link to "email" (this one:
http://portal.plus.net/general/webmail.html) takes you to a login - but NOT for webmail. It's the login for the portal, so of course user+mailbox doesn't work. In the address bar is "http://portal.plus.net/error.html"
This is using firefox. Try it in IE and it works OK.
Any reason for this, apart from my gut instinct that wouldn't make family reading?
And now can't get to the help and support pages...