When you have a look via the browser’s developer tools (F12 in Chrome i.e., Network tab), you can see that this URL (I changed it to not contain sensible data) is causing the 404:
https://your.mailserver.tld/admin/inc/ajax/transport_check.php?transport_id=1&transport_type=transport-map&mail_from=test%40test.de&mail_rcpt=null%40hosted.mailcow.de&csrf_token=XXXXXXX
The problem here is the new /admin prefix of the URL, it seems the Mailcow team didn’t adjust it accordingly. If you copy the URL, remove “/admin” from it and use the result in your browser, it works.