At https://docs.redhat.com/en/documentation/openshift_container_platform/4.17/html/installing_on_a_single_node/install-sno-installing-sno#generating-the-install-iso-manually_install-sno-installing-sno-with-the-assisted-installer  creating a Single Node OpenShift ISO installation image is described.

There should be a note that the image is only usable for 24 hours, because it includes temporary SSL certificates valid for 24 hours. To re-create the image after that time, delete the "ocp" directory and repeat everything from Step 8.

Thanks to itsoiref@redhat.com for pinpointing this issue.