There are two common issues. 


Firstly make sure you are logging in through the log in link and not through the sign up form.


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Secondly please make sure that arrive on this page and enter your email address and password.


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Finally If after entering your password you get the following fault


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It means you have used an illegal or incorrect symbol or character in your email. For example you have used ^ or a % or something which is not part of an email. Check that you are using @ rather than any other symbol in the middle of your email.