Ubuntu 20.04 / 21.04 stuck in grub after encryption
Solution 1:
The problem originates from large initrd.img files (~100MB) generated with MODULES=most that can't be loaded due to size limitations. This can be solved by switching to MODULES=dep which generates initrd.img files of approximately 55MB.
See this bug report: https://bugs.launchpad.net/bugs/1931024