I did it... :)
Latest nightly from apt-repository was installed (4.1xxx), running and working
Clone the git repo
run Autoinstall.sh (installed missing packages and rerun the script)
wait...
wait...
wait...
over two hours later the build was done
install the generated package
Check tvheadend about-site
Shows 4.2114 - looks good
watch some channels with tvhclient - looks also good. :-)
So it seems that the switch from apt-repository to own build works as expected but takes a very long time. :-)