I don't have enough reputation to answer any question, so I decided to create a new one to share a solution to my problem.
In my case, I have a dual boot with Windows 10 and Ubuntu. I did an automatic upgrade of my Kubuntu from 20.04 to 22.04. Once the process finished, I got the error "out of memory" error when loading initial ramdisk that leads to "Kernel Panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)" when booting to my Kubuntu partition.
I followed this thread: Kernel Panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0) without success. They focused mainly in a messy upgrade process, but in fact my problem was with the ESP partition size that was not able to load initramfs. Luckily, my /etc/initramfs-tools/initramfs.conf was configured to load all the possible modules, so I modified the variable MODULES from most to dep following this thread: https://bugs.launchpad.net/ubuntu/+source/initramfs-tools/+bug/1970402.
MODULES=dep
Now, the error disappeared and I can boot normally. Is it possible that during the linux upgrade a new ESP was created? In fact, I created initially a ESP partition with more than 300MB when installing the dual-boot to have enough space. I should have performed manually the upgrade instead of relying on the automatic upgrade...
