Incoming Requests are not tracked

I have added a new site to my Matomo tracking, but in the dashboard doesn’t show any visitors.
I verified, that visitors are sending requests to the server as I can find test requests in the access logs.
Also the piwiki.js file is downloadable on the client side.
Requests look very similiar to these of other sites (that show data btw).

Matomo-Version: 3.13
PHP: 7.3.29-1
Mysql: 10.3.29-MariaDB

Did someone encounter this before or can give me a hint on how to resolve this?

Maybe have a look at:
Http api need help - Support & Bugs - Matomo forums