If you go to www.horando.de you can see that the server currently does not interprete css and js files. Whats wrong here with the server?
Everything went smooth 10 minutes ago, no deploys or anything?!
My site also has the same issue only very recently, causing browsers to refuse to accept the CSS. Likewise for Javascript modules.
The stylesheet https://leapstone.netlify.app/_app/assets/pages/__layout.svelte-9c4e6dbf.css was not loaded because its MIME type, "text/plain", is not "text/css".
Note that with DevTools enabled the page will still load. Possibly something to do with the service worker?
Checking on the command-line I do see the CSS is returned as plain text:
❯ http --print=h get https://leapstone.netlify.app/_app/assets/pages/__layout.svelte-9c4e6dbf.css | grep Content-Type
Content-Type: text/css; charset=UTF-8
I can see both working fine now. Are you still having issues?