OK I have one more piece of the puzzle and now it seems to work (again)
This instruction says for newer versions of MacOS you need an additional installation
https://cran.r-project.org/doc/manuals/R-admin.html#macOS
sudo installer -pkg \ /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg \ -target /
After I did this I was able to install RServe via the ‘regular’ command. As far as I understand the description this command makes certain paths/headers ‘known’ to the tools. Now I get a lot of warnings but in the end the compilation is done. I will have to investigate if it is feasible to provide a compiled version somewhere.
And also to bring all this together in a description …