Sometimes the activation links can get cut off depending of the email program and the settings you are using. Thus leaving a part of the URL out that is required for activation. You should be able to fix the problem by manually copy pasting the URL to your browser.
Similar errors can also occur if a some days have already passed since you received the confirmation email, resulting in the expiration of the invitation.
In this case, you should re-register using the same details as previously. Should that fail for some reason, you can contact us for further support.
You can read more about registering and logging in here.