I have just had a call with IceSave, with whom I have opened a new savings account. I have been consistently unable to log in and so wanted to work through the error. The nice girl on the phone confidently told me that all she needed to do was ‘refresh the internet’ for me and it would work tomorrow.
Update 7th Feb: Incredible. It works. Next time you’re suffering login problems, I suggest you try refreshing the internet. It worked for me.