Always triggering 404 of main site

Hello,

I have my main instance of WordPress in the root directory of my webserver (https://xyz.com/). Now I installed a second instance for a customer of mine (https://xyz.com/second/).

The home page is working but if I navigate to another page (https://xyz.com/second/specific-page/) I’m triggering a 404 error on my main instance. I understand why this is happening but what can I do to prevent this behaviour?

Thanks in advance.