The operating system my web server runs on is (include version):
Debian 10 Backports
I can login to a root shell on my machine (yes or no, or I don't know):
yes
I'm using a control panel to manage my site (no, or provide the name and version of the control panel):
no
You are using port 444 in your https connection for domain vreecoaster.de and port 445 for discord.vreecoaster.de and you should use port 443 for both.
An example, use below url and you will access your site correctly:
In inkognito mode I can join, I deleted cache and cookies but the index.php get downloaded on join
EDIT: It works now, don't know why. Very thanks for your help, I really appreciate it!