Member Registration information issue

Good day. My forums' registration is set to email validation.
But after they click the email link for validation they get this information:

Sorry, but no such user exists.

Can any of you help me fix this?

Thanks.

make sure they have only clicked it once, double clicking will activate it then tell them the user doesnt exist because its already been activated. But to check that this is the case you can go into admin>>>users and groups>>>inactive users. If the member is still there, you can activate them there.