With the new Ubuntu autoinstaller, some changes in the kernel files (path and file name) have been introduced as well. When unpacking the .iso image, it provides vmlinuz and initrd instead of linux and initrd.gz as before.
The operating system type Debian (which is used for Ubuntu as well) must be configured to detect the new files.

More details:

• https://community.theforeman.org/t/autoinstalling-ubuntu-server-20-04-3-from-the-live-iso/27050/14
• https://projects.theforeman.org/issues/32632