Ubuntu fails to start by default after upgrade 18.04 to 20.04

What exactly happens when you try booting normally?

Have you tried booting in "advanced - older kernel" and then re-installed the grub and kernel packages?

booting normally => system hangs already on BIOS screen.
reinstallation of grub as indicated on https://askubuntu.com/questions/1338236/ubuntu-fails-to-start-by-default-after-upgrade-18-04-to-20-04 didn´t solve the issue. Also I assume I am using grub2 already, but not 100% sure.
No idea, thus any hint appreciated.
Thanks, Joachim

For diagnostic purposes, what is the output of the commands

uname -a
lsb_release -crid
sudo dpkg --audit
dpkg -l | grep grub

(You may redact proprietary information if you deem that necessary)

root@letsencrypt:~$ uname -a
Linux letsencrypt 4.15.0-143-generic #147-Ubuntu SMP Wed Apr 14 16:10:11 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
root@letsencrypt:~$ lsb_release -crid
Distributor ID: Ubuntu
Description: Ubuntu 20.04.2 LTS
Release: 20.04
Codename: focal
root@letsencrypt:/home/joachim# dpkg --audit
root@letsencrypt:/home/joachim# dpkg -l | grep grub
ii grub-common 2.04-1ubuntu26.11 amd64 GRand Unified Bootloader (common files)
ii grub-efi-amd64 2.04-1ubuntu44 amd64 GRand Unified Bootloader, version 2 (EFI-AMD64 version)
ii grub-efi-amd64-bin 2.04-1ubuntu44 amd64 GRand Unified Bootloader, version 2 (EFI-AMD64 modules)
ii grub-efi-amd64-signed 1.167+2.04-1ubuntu44 amd64 GRand Unified Bootloader, version 2 (EFI-AMD64 version, signed)
ii grub-legacy-ec2 1:1ubuntu1 all Handles update-grub for ec2 instances
rc grub-pc 2.02-2ubuntu8.2 amd64 GRand Unified Bootloader, version 2 (PC/BIOS version)
ii grub2-common 2.04-1ubuntu26.11 amd64 GRand Unified Bootloader (common files for version 2)

No idea what the output were in case I´d pick the 5.4 kernel during boot..
Thanks, Joachim

Your output confirms that you are running grub2.

"system hangs already on BIOS screen."
If the system is really hanging during the BIOS start phase, then this is not an Ubuntu problem, because Ubuntu is started only after BIOS.

I disagree. The BIOS screen of Hyper-V is just shown until it is replaced by something else - and before the upgrade or with kernel 4.15 the screen is replaced with startup messages. This just indicates the upgrade caused an error very early in startup.
And whether it is BIOS, kernel, or upgrade - I don´t know.
Thanks, Joachim

Can you describe what exactly happens when you force the grub menu to show when booting, and then select to boot "Ubuntu"?

when I go via Grub Menu and select first "Advanced options for Ubuntu", then "Ubuntu with Linux 5.4.073-generic" (this is the default)
then I get

Loading Linux 5.4.0-73-generic ...
Loading initial ramdisk ...

and then system dead. Actually the BIOS screen disappeared this time, but I have seen other instances that left it in place, and also no message. I guess this was a good take however.

Same sequence with "Ubuntu with Linux 4.15.." and the system boots successfully.

This may indicate that there is something wrong in the kernel files for booting.

My suggestion:
Boot into the 4.15 kernel and reinstall all kernel packages from version 5.4.0-73-generic

The command
dpkg -l | grep 5.4.0-73-generic
will list them, and then execute the command
sudo apt install --reinstall linux-image-5.4.0-73-generic linux-modules-5.4.0-73-generic linux-modules-extra-5.4.0-73-generic
(Adapt the list to match what you already have installed)

tried that, including headers, but system still does not boot with 5.4.

I am sorry, but I am at my wits' end.

Maybe future kernel versions may help, there is already 5.4.0-74 in focal-proposed.

And a complete different approach would be to switch to the hwe kernel series (currently 5.8.0-54) by installing linux-generic-hwe-20.04

I tried hwe kernel right now and same problem. Also I don´t really think newer (virtual) drivers are required for Hyper-V.
Any idea how to proceed? A bug report? what component?
Thanks, Joachim

If you already have done all suggestions that are listed in the askubuntu question (update-grub, grub-install, etc.), then I do not know what else can be done.
For creating a bug report I would have suggested the kernel packages "linux" as target, but if you have the same effects also with hwe, then the culprit may be somewhere else.