Meanwhile, I got it up and running. I do not even know exactly how but I guess that it has something to do with the installation of a wrong OPAM package, maybe even through the Centova Cast installation script.
But after deleting the two OPAM folders in the "root"-folder and then downloading the current packages via Debian command, as suggested on the OPAM website, the installation worked flawlessly via its update feature.
apt-get install opam
After that I did the installation of Liquidsoap again via the installation script, this time without error messages.
/usr/local/centovacast/sbin/update --add liquidsoap
After that, I was able to use Shoutcast 2 with Liquidsoap as Auto DJ.
So it seems that some users have to install OPAM via distribution's package system instead with the automatic OPAM script which was also not working for me.
Maybe this info page will also help other users who have similar problems:https://opam.ocaml.org/doc/Install.html