crypttab seems to not been active
After I upgraded from Linux Mint 21.1 to 21.2 the newest initrd stopped working. The older one works fine. Regenerating the initrd doesn’t help. On startup with the newest initrd the error tells that the UUID of my decrypted Rootdrive does not exist, so I am guessing that the crypttab doesn’t get used. I checked the crypttab that hasn’t changed.
If your root filesystem is encrypted, in modern Debian/Ubuntu/Mint you will need not only the cryptsetup package, but also the cryptsetup-initramfs package.
After the upgrade, your /var/cache/apt/archives/ was most likely full of downloaded packages, and so it might have happened that the system is running out of disk space when regenerating the initrd.
This typically causes the initrd creation to fail, possibly causing a partial initrd file to be created. Trying to boot with an incomplete initrd file could easily cause the failure you’re seeing.
So first, run ls -l /boot and look at the sizes of the initrd files. If the initrd of the new kernel is significantly smaller than one for older kernels, it is probably missing some parts.
In that case, try sudo apt clean all to make some space by cleaning the package cache (always a good idea after a major update, if disk space is tight), then make sure that the cryptsetup-initramfs package is installed and is up to date. Then try regenerating the initrd for the new kernel again.