Its finally working.. pff not really much help here but that's probably just me.
The problem was with the current stable version (18.104.22.168) that you can bring the Apache server down by signing a wrong .crt.
So before, I deleted everything related to SSL to be able to start the Apache server again. But I also deleted
I replaced that rule and everything is working fine now! And also, you shouldn't choose an IP in sites in ISPConfig, just leave the *.