That's strange, as far as I know everything worked fine for me (or actually still does so on my old machine).
Well I tried to install LC in a Manjaro VM, however the installer stopped at some point with some "payload" message. Really frustrating. So I copied the .runrev folder from my old (RebornOS) machine but that doesn't do anything. Problem remains, that nothing Livecodish runs here under EndeavourOS.I wonder what would happen if you were to install Livecode on a VM in a supported distro - where you can "expect" it to work - and then move the .runrev directory from the VM to your base machine. It's a bit of extra work but it may give you the result you want.
So at the moment it looks like farewell Livecode. Up to now I am very pleased with EndeavourOS/Xfce and will not trade this OS for one single program that doesn't run. Thank you everybody for trying to help and don't feel too safe from future stupid questions from me (regarding Livecode programming), since I do have it running on the old machine .