Error upgrading Ubuntu armhf from 18.04 to 20.04 on RPI4 - package linux-firmware 1.187.9 failed to install/upgrade

[Avio]

Error upgrading Ubuntu armhf from 18.04 to 20.04 on RPI4 - package linux-firmware 1.187.9 failed to install/upgrade: il sottoprocesso installato pacchetto linux-firmware script post-installation ha restituito lo stato di errore 1

This should be the most relevant part:
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Configurazione di linux-firmware (1.187.9)...
update-initramfs: Generating /boot/initrd.img-5.3.0-1008-raspi2
Using DTB: bcm2711-rpi-4-b.dtb
Installing /lib/firmware/5.3.0-1008-raspi2/device-tree/bcm2711-rpi-4-b.dtb into /boot/dtbs/5.3.0-1008-raspi2/./bcm2711-rpi-4-b.dtb
Installing new bcm2711-rpi-4-b.dtb.
Ignoring old or unknown version 5.3.0-1008-raspi2 (latest is 5.4.0-1028-raspi)
update-initramfs: Generating /boot/initrd.img-4.15.0-1073-raspi2
Using DTB: bcm2711-rpi-4-b.dtb
Couldn't find DTB bcm2711-rpi-4-b.dtb on the following paths: /etc/flash-kernel/dtbs /usr/lib/linux-image-4.15.0-1073-raspi2 /lib/firmware/4.15.0-1073-raspi2/device-tree/
Installing into /boot/dtbs/4.15.0-1073-raspi2/./bcm2711-rpi-4-b.dtb
cp: cannot stat '': No such file or directory
run-parts: /etc/initramfs/post-update.d//flash-kernel exited with return code 1
dpkg: errore nell'elaborare il pacchetto linux-firmware (--configure):
 il sottoprocesso installato pacchetto linux-firmware script post-installation ha restituito lo stato di errore 1
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ProblemType: Package
DistroRelease: Ubuntu 20.04
Package: linux-firmware 1.187.9
Uname: Linux 5.10.9-v7l+ armv7l
ApportVersion: 2.20.11-0ubuntu27
Architecture: armhf
CasperMD5CheckResult: skip
Date: Tue Jan 26 14:40:00 2021
Dependencies:

ErrorMessage: il sottoprocesso installato pacchetto linux-firmware script post-installation ha restituito lo stato di errore 1
PackageArchitecture: all
Python3Details: /usr/bin/python3.8, Python 3.8.2, python3-minimal, 3.8.2-0ubuntu2
PythonDetails: N/A
RelatedPackageVersions:
 dpkg 1.19.7ubuntu3
 apt 2.0.2
SourcePackage: linux-firmware
Title: package linux-firmware 1.187.9 failed to install/upgrade: il sottoprocesso installato pacchetto linux-firmware script post-installation ha restituito lo stato di errore 1
UpgradeStatus: Upgraded to focal on 2021-01-26 (0 days ago)

[Juerg Haefliger (juergh)]

It seems flash-kernel is confused about what DTB to use. No wonder given all the changes with the Pi DTBs :-(