Hello Otared,
Thank you very much for your attention and your hints, which solved my problems with ConTeXt from TeXLive2020.
Indeed following your suugestion, I removed the /opt/local/etc/texmf/ directory, as well as all other tex related directories and files in /opt/local/, and then installed again MacTeX2020. In oder to be on the safe side, I rebuilt also the locate database, then rebooted my machine: after that I could run luatools --generate and now with context --version I have current version: 2020.03.10 14:44.
However I don't know why I had tex related files in /opt/local/, which is used by MacPorts…
Some programs have dependencies on texlive. These are usually programs that create pdf via TeX. Glad I could help you. Greetings Axel