You need to reschedule the exam again in the certification page, this button is valid for just one attempt!
They will send to you another email with another link to access, just remembering that you have only two attempts with the free voucher for each certification (intermediate and advanced).
Is in the Mail also a Button to check out the Browser Settings? Maybe the button below to start is only visible when you complet the Browser Test.
If not contact the Mendix Support :-)