Dear @Axel, my SDK directory looks like this:
lrwxr-xr-x 1 root wheel 14 16 Dez 15:01 MacOSX.sdk -> MacOSX11.1.sdk
drwxr-xr-x 8 root wheel 256 9 Jul 2020 MacOSX10.15.sdk
drwxr-xr-x 7 root wheel 224 30 Nov 13:33 MacOSX11.1.sdk
but root 6.22/06 searches explicitly for MacOSX11.0.sdk. It seems that the new numbering scheme from Apple created some mess here. Creating a symlink solved the problem for me but others might run into the same issue:
sudo ln -s MacOSX.sdk MacOSX11.0.sdk