I've just tried logging in and got the following message ....
THERE IS A SERVER PROBLEM! PLEASE LOGIN LATER.
I had this yesterday for a while too but tried about an hour or so later and it was all sorted. It may be that they are updating something on the exercise page? Anyway, you're not alone, just try again later is my advice 
If at first you don't succeed, try try try again ;-)
'My Dore' Webpage - http://my.dore.co.uk/Default.aspx
Helen, Mum to 13yr old Danielle - 11 months into Dore - mild dyslexia!