Hi,
I didn't get a 404 error with that link on your website.
It worked fine and I got the message:
Ihr Benutzerkonto wurde erfolgreich aktiviert! Sie können sich jetzt mit dem von Ihnen bei der Registrierung gewählten Benutzernamen und Passwort anmelden.
Der Warenkorb ist leer
and was redirected to the products listing (which is normal since I didn't add anything to the cart before clicking on the link and thus the checkout redirected me to the products listing).
Are you able to reproduce the problem ? Maybe you clicked on the link after changing something else on the website ? Or you changed something since then which fixed the issue and thus I didn't see it ?