If you go into your account settings, there should be a button to verify your account. I am not exactly sure where, but I would assume it is under 'Email adresses', as you will see a 'Verified' otherwise. If you see that, you already have verified it and that is why it no longer works. Otherwise, try verifying once more via the button. Going from your description I would guess that the e-mail you have may not be 'usable' anymore.
Viewing post in Unable to verify email
You would be right in that regard. However, from how they described it, it sounded like they are having a problem with the activation link working within the first e-mail they had received.
So in that regard, the error they received would either come from the link being outdated, as it may only work for a certain time frame, or because their account had already been verified.
In both cases, the recommendation would be to locate the button within the account settings to check the verification status and/or send a new e-mail.