I was able to resolve the problem. Unfortunately, I did not find out what exactly caused the strange issue. Most probably it was a mix of my project and the server configuration. I never had something like this before.
To come to a solution, I used a “fresh” starter kit and updated it with my files. Then I did several “server” setups with Docker and my project, simulating a similar environment.
Starting with Apache, I moved to NGINX and finally to Caddy. This worked with Docker and the project, and it also seems on the cloud server.
Thank you for your support!