We tried to install RHEL 8.8 on one of our Fujitsu Synergy RX300 S8 but media fails to boot giving this error:

Invalid image Failed to read header: Unsupported

Failed to load image: Unsupported

start_image() returned Unsupported

BIOS is configured in UEFI mode without secure boot

After some research we reconducted this issue to a shim bug and tried to boot all RHEL 8.x backwards.

This Fedora bug seems to match:

https://bugzilla.redhat.com/show_bug.cgi?id=2113005

It finally worked with rhel 8.5 that has shim 15.4 so both shim 15.5 and 15.6 are affected moreover we also have rhel 7.9 on some of this servers and it has shim 15.0 but an update to 15.6 is available so we expect the same bug