Error 404 only on some clients

I am routing incoming traffic to different containers. Since I am behind a dynamic IP address I configured my domaind to use a CNAME record to access my server using a DDNS service. For the most of my configured URLs and containers everything works perfectly.

But there is one strange behaviour: For the one or the other URL I will receive a 404 error - but not on all my client laptops. From some client machines I can access the website in the specific container, but from other machines not.
It does not matter if the machines are in different WiFi networks or even completely external from my network. Furthermore when trying to access the service from a machine that results in a 404 error I do not see any information in the traefik logs.

Any idea how I can troubleshoot that further?