Page and panel 404 error after server upload

Hello there,
I am working on a (semi-professional) website for a “client” and for testing it on a server before sending it to them I installed it quickly on my own server and everything worked. Now I sent it to them, they uploaded it to their server (which I unfortunately have no access to) and the website displays a 404 error. A few days ago we uploaded an earlier version of the site but because we ran into a few problems (that were always displayed with the debug menu and overall the site worked) we took it down again, that time the panel was also accessible, now it has also a 404 error.

The site is raingers.ai in case anyone can see more than me. The exact same test site on my server is on ulmer.ink for which my panel also works. Between the two versions the only big change was adding the multi language feature. Also my client apparently doesn’t have the .DS_store file in the root that shows up on my server (but doesn’t show up in my finder although I can see the other hidden files).

I’m not sure what information is helpful but I’ll be happy to post it and also grateful if anyone can help me! What I do know is that the Kirby version is 4.0.2.
Thanks in advance!

That looks like a misconfiguration of the server or missing files (.htaccess not uploaded, for example).