Installing macos El Capitan in Parallels on a Mac

Hi,
I upgraded my older Parallels license to Parallels 17 a while back to run macos 10.11 on my 2019 macbook Pro under Catalina.
After many failed attempts I found an explanation: The Parallels Capitan image can only run on hardware that will run El Capitan natively.
So, many weeks later, I finally have potent hardware (a trash can Mac Pro) that will run El Capitan natively.
Now I try Parallels 17 again. But I get the same result: The boot process shows a white Apple on black background. The progress bar goes about half way through and then it stops with a no parking sign. This result is the same regardless of whether I use the Recovery Disk method, whether I let Parallels create an installation image from the 2025 Apple installer or whether I use a fully bootable disk image created (and verified) using Carbon Copy Cloner.
I searched around support forums both here and on other internet sites.
It seems there are people that simply install and it works. And many frustrated people like myself where it simply fails.
So, anyone, what is the magic trick to have this working?