I've had the same issue. In fact, I can't get any kernel after 4.15 to boot after initial installation.
I have the same issue. I get a boot error: efi rng protocol not available There's a kb article about how disable EFI (https://kb.parallels.com/en/122735) but even after doing that the error persists. The only thing that works is booting with an older kernel version.
I have posted the reason for this in the below thread https://forum.parallels.com/threads/official-ubuntu-20-04-live-server-does-not-boot.356891/ I've also tried to communicate this by raising a support request however I cannot get past the first line people who do not understand the issue.