I'm also experiencing this after the latest 21.10 update on two builds on the same Raspberry Pi 400, booting from USB. I added a comment on https://bugs.launchpad.net/ubuntu/+source/linux-raspi/+bug/1937924 which sounded similar. I have restored a bootable system by mounting the devices on another machine and replacing initrd.img and vmlinuz on the boot partition with the .bak versions.
I'm also experiencing this after the latest 21.10 update on two builds on the same Raspberry Pi 400, booting from USB. I added a comment on https:/ /bugs.launchpad .net/ubuntu/ +source/ linux-raspi/ +bug/1937924 which sounded similar. I have restored a bootable system by mounting the devices on another machine and replacing initrd.img and vmlinuz on the boot partition with the .bak versions.