I have no idea what will happen if you simply comment the themes-line from /etc/default/grub. That may cause a real hickup...

Commenting that line simply disables the fancy grub in favor of the always good and old fashioned grub... 😉

I wonder if this should be reported as bug... 🤔

The solution is to copy the /usr/share/desktop-base/grub-themes/desktop-grub-theme-files to /boot/grub/themes. This will provide the default layout in an unencrypted location. Every now and then the /etc/default/grub may get overwritten by updates. Then you have to fiddle again.

I have no idea what will happen if you simply comment the themes-line from /etc/default/grub. That may cause a real hickup...

This line

Found theme: /usr/share/desktop-base/grub-themes/desktop-grub-theme/theme.txt

indicates that grub wants to read the path /usr/share/desktop-base/grub-themes/desktop-grub-theme. This path is available on an unencrypted system for grub and for the system when running update-grub at any time, no errros.
On an encrypted system, when grub starts, this path isn't available yet until the initramfs has opened the root directory. This is when you get the errors as from the initial post. When running update grub there is no error since then /usr/share.... is available.

Half understood... /boot is actually clear...

Are you tell me to update grub configuration and move the theme into /boot...

I don't need a theme at all how can I remove it? I prefer plain old-style grub perhaps this is even better to solve it... 🤔

Found theme: /usr/share/desktop-base/grub-themes/desktop-grub-theme/theme.txt

indicates that grub wants to read the path /usr/share/desktop-base/grub-themes/desktop-grub-theme. This path is available on an unencrypted system for grub and for the system when running update-grub at any time, no errros.
On an encrypted system, when grub starts, this path isn't available yet until the initramfs has opened the root directory. This is when you get the errors as from the initial post. When running update grub there is no error since then /usr/share.... is available.

The installation of the clearlooks-phenix-sapphire-theme changed /etc/grub/default back to desktop-base and the themes there. On my encrypted Daedalus this produced the initial error in grub, an update-grub did NOT fix this. I copied clearlooks-phenix-sapphire-theme to /boot/grub/themes and changed the  /etc/grub/default to point to the /boot/grub/themes-directory, and everything is fine again.

Mi issue came back...

However when I update-grub I don't see any complaining...

sudo update-grub
[sudo] password for freezr: 
Generating grub configuration file ...
Found theme: /usr/share/desktop-base/grub-themes/desktop-grub-theme/theme.txt
Found background image: splash.png
Found linux image: /boot/vmlinuz-6.1.0-7-amd64
Found initrd image: /boot/initrd.img-6.1.0-7-amd64
Found linux image: /boot/vmlinuz-6.1.0-6-amd64
Found initrd image: /boot/initrd.img-6.1.0-6-amd64
Warning: os-prober will not be executed to detect other bootable partitions.
Systems on them will not be added to the GRUB boot configuration.
Check GRUB_DISABLE_OS_PROBER documentation entry.
Adding boot menu entry for UEFI Firmware Settings ...
done

ufff... 😖

Good you fixed it

It doesn't matter now, but I'm not sure what you meant with "Already posted"? Afaict your OP doesn't include output of either ls -l /dev/disk/by-uuid (note the -l) or blkid, which both are ways of seeing the associations between uuids and partitions.

I posted the first command but not the second... And actually the partition it was unable to find was the LVM one belonging to root...

/dev/mapper/abook--vg-root: UUID="3ba85e43-bd5b-4a44-9544-41921221101b" BLOCK_SIZE="4096" TYPE="ext4"
/dev/mmcblk0p3: UUID="67af2c73-280d-49f1-aef0-5ed416c42ab8" TYPE="crypto_LUKS" PARTUUID="d37ff9cc-dde0-4633-9a1b-276f81d36161"
/dev/mmcblk0p1: UUID="F447-1A01" BLOCK_SIZE="512" TYPE="vfat" PARTUUID="6391c5b8-f0a0-48f9-bca4-ecbb6a9869de"
/dev/mmcblk0p2: UUID="fa11faec-6990-4e47-b7da-2deba175d392" BLOCK_SIZE="1024" TYPE="ext2" PARTUUID="60c53789-c48d-4052-956d-99a6419a97eb"
/dev/mapper/abook--vg-swap_1: UUID="d1ad949a-3157-4090-b03f-f8fb51c3816f" TYPE="swap"
/dev/mapper/mmcblk0p3_crypt: UUID="SXoUom-lUES-U2K6-Acug-MJDg-1t1P-kudC1C" TYPE="LVM2_member"

It doesn't matter now, but I'm not sure what you meant with "Already posted"? Afaict your OP doesn't include output of either ls -l /dev/disk/by-uuid (note the -l) or blkid, which both are ways of seeing the associations between uuids and partitions.

Could it be that grub searches for the theme on the encrypted system device? Check /etc/default/grub.

When this is the case you may need to move the theme files to the unencrypted /boot.

This sound interesting, actually the theme as changed.

However /EFI and /BOOT are over clear partitions...

From /etc/default/grub:

GRUB_THEME=/usr/share/desktop-base/grub-themes/desktop-grub-theme/theme.txt

Mmm... 🤔

Kind of looks like something pre-grub tries to access the partition while the partition uuid is encypted.

Maybe ls -l /dev/disk/by-uuid or blkid would help identifying which partition it concerns.

Already posted!

When this is the case you may need to move the theme files to the unencrypted /boot.

Maybe ls -l /dev/disk/by-uuid or blkid would help identifying which partition it concerns.