If you try to look at this card while not logged in, you get a 404 error. This is fine, as it masks whether cards exist, but there's no way from that page to easily log in if you know the card does exist.
Couple of potential solutions:
Redirect any page that's not found to the login page if you're not logged in, then once you're logged in redirect back to the page you were on. If the page genuinely doesn't exist at that point then you get the 404
Add a log in form to 404 pages (or a link to the log in page, like GitHub does)
If you try to look at this card while not logged in, you get a 404 error. This is fine, as it masks whether cards exist, but there's no way from that page to easily log in if you know the card does exist.
Couple of potential solutions: