I've upgraded two laptops from ubuntu- mate 23.10 to24.04. Both laptops have nvme ssds formatted ZFS with encryption. After the installation and reboot, appears that grub gets hung up on a gray screen. If I press esc key and select to boot to 24.04 but use the 6.5 kernel instead of 6.8, both laptops will boot and let me log in. Is anyone else having issues with the 6.8 kernel not working with encrypted ZFS after doing an upgrade?
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Hi, @mjb32803 (Mike) 
(Usual disclaimer: please note that I'm just another Forum user here. I'm NOT a Ubuntu developer or Ubuntu MATE developer)
There are several bug reports about Ubuntu 24.04 LTS ("Noble Numbat") booting into "black screen" / "blank screen", even without using ZFS (encrypted or not):
You may want to change some GRUB (GRand Unified Boot Loader) settings that include removing "quiet splash" from the GRUB settings as I've explained in the following topic (as this seems to solve the problem for some people):
If that (alone) doesn't work, you may also want to configure GRUB so it only boots to the older working Kernel as, for instance, @jymm (Jim) has explained in the following reply:
I hope this helps Please reply back later in this same discussion topic that you've opened, to tell us what worked for you or not, as that can be very useful for other users.
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Thanks for all your input. I think the most applicable one you posted was "Green screen after update." I went in and edited /etc/default/ grub with the recommended changes. Rebooted and at least now I can see the error message. Letting it boot normally using the 6.8 kernel, I'm getting handful of errors right before the boot process stops at the "(initramfs)"prompt.
First error is
Key load error: failed to open key material file: no such file
Begin: Mounting 'rpool/ROOT/ununtu_m5runx' on 'root//' ... Failure: 2
Then I have a few more lines showing errors also trying to mount rpool and wants me to manually Mount the file system and exit.
Eventually BusyBox kicks in and drops me to the (initramfs) prompt, where I assume I'm supposed to manually mount rpool and then maybe the boot process continues?
This is all a little bit beyond my technical understanding as I have very limited experience doing this type of debugging and I've only used ZFS for about 3 months.
Any insight would be appreciated in the meantime I'll start doing some googling on the error messages.
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Hi again, @mjb32803 (Mike)
Well, based on those boot messages, it seems that indeed the problem you're having, when booting up from the 6.8 Linux Kernel in Ubuntu MATE 24.04 LTS ("Noble Numbat") is related to ZFS Encryption.
Unfortunately, I don't have experience with ZFS (and, by extension, with Encrypted ZFS). I did find the following related Bug, with the same error messages that you got, that was reported in Launchpad, by user geole0, on 24th February 2022, for a pre-release version of Ubuntu 22.04 LTS ("Jammy Jellyfish"):
Bug #1962061 “ZFS Key load error: Failed to open key material material file: No such file or directory” : Bugs : systemd package : Ubuntu
https://bugs.launchpad.net/ubuntu/+source/systemd/+bug/1962061
From that bug report:
" (...) The message is
Key load error: Failed to open key material file: No such file or directory
Command: mount -o zfsutil -t zfs rpool/ROOT/ubuntu_ui69ph /root// > Message: filesystem 'rpool/ROOT/ubuntu_ui69ph' can not be mounted: Permission denied
filesystem 'rpool/ROOT/ubuntu_ui69ph' can not be mounted: Permission deniedHello
I recently installed an encrypted ZFS 22.04 version. (...)"
The same user reported, on that same Bug web page, that the error disappeared when using later Daily builds (in ISO format) of pre-release versions of Ubuntu 22.04.
My suggestion, @mjb32803 is that you open a new Bug Report for the "zfs-linux" Ubuntu software package in Launchpad, detailing your scenario (among other things, I suggest that you mention that it was the result of an upgrade from Ubuntu 23.10 ("Mantic Minotaur") to Ubuntu 24.04 LTS ("Noble Numbat") and that you also include the information that it boots well when choosing the 6.5 Linux Kernel but gives errors when using the 6.8 Linux Kernel) and the error messages that you got. The "Bugs" web page of "zfs-linux" ordered by age (most recent bugs first) is the following:
https://bugs.launchpad.net/ubuntu/+source/zfs-linux/+bugs?orderby=-datecreated&start=0
And here is the link to report a new bug for the "zfs-linux" Ubuntu software package:
https://bugs.launchpad.net/ubuntu/+source/zfs-linux/+filebug
I hope this helps, at least somewhat. Good luck! Please, keep us posted about your situation.
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Got exactly the same issue. Would be great if you could share your solution 
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Welcome @M_H to the community!
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Thanks mate
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I never did find a solution. I posted a bug report under ZFS but it just sat there and never got looked at by anybody. Eventually I just ended up reinstalling 24.04. from scratch and got rid of ZFS and switched back to LUKS with encryption. I actually spent less time reinstalling and reconfiguring two computers then I spent trying to debug this issue. As I mentioned in my original post, it's definitely something to do with the 6.8 kernel because if you boot using the 6.5 kernel from 23.10, everything seems to work okay. I guess I should have taken it more serious when You go to install ZFS and it clearly states that it is an experimental feature. I just wouldn't use it anymore.
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Damnnn... This is my thought process at the moment as well. Re-installing.
Thanks for getting back to me. Sadly I do not have a working kernel as an option to boot. So I guess I am screwed. At least I will start fresh I suppose.
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