No thats not the case… If I have no idea at this moment, then I don’t answer. And it is bad if only one person answers. More heads produce better results… (something like that). So @andreas85 explained it in an excellent way.
So on /boot there are initramfs. These initramfs are not rolled back and therefore it crash. I guess an easy fix could be:
mkinitcpio -P
update-grub
You do all right, in only following one advice at a time! megavolt is very competent like a lot of members in this forum. I must be honest. you wrote the most of this in your first post. But i did not recognize some of it
So what do we know until now:
Lets assume that “Failed to start Load Kernel Modules” is because the Kernel-Modules on /lib/modules don’t match the kernel in /boot
There seem to be several ways to rescue your install:
After this i did remove the /boot and included it into /
This is indeed the first(best) way to go. It may be additionally necessary to replace the kernel with mkwd-kernel so that kernel and /lib/modules/* have the same version.
I don’t think so… Let’s assume he have installed an update. It updates the kernel, creates a new initramfs image and updates grub.
Now he rolled it back. But since /boot is not on the same partition (and therefore not included in the snapshot), it is not rolled back and stays the same. Now the new images after update stay and everything else is rolled back… So if he install a new kernel with mhwd, then the initramfs image will be recreated anyway, but the other stay.
Oh man… that is getting complicated…
On BTRFS /boot should be included and not a separated partition. Less headache, when using snapshots. Fully agree here with @andreas85 
I think in /lib/modules on the disk there may be an old version of the modules from the rollback. When mkinitcpio runs it will try to use these. but the kernel is an newer version. At the moment the kernel and the initramfs may be the same version.
But You may be right. And it does not harm to try this. There will be no damage
[manjaro /]# mkinitcpio -P
==> Building image from preset: /etc/mkinitcpio.d/linux419.preset: 'default'
-> -k /boot/vmlinuz-4.19-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-4.19-x86_64.img
==> ERROR: specified kernel image does not exist: `/boot/vmlinuz-4.19-x86_64'
==> Building image from preset: /etc/mkinitcpio.d/linux419.preset: 'fallback'
-> -k /boot/vmlinuz-4.19-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-4.19-x86_64-fallback.img -S autodetect
==> ERROR: specified kernel image does not exist: `/boot/vmlinuz-4.19-x86_64'
==> Building image from preset: /etc/mkinitcpio.d/linux510.preset: 'default'
-> -k /boot/vmlinuz-5.10-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.10-x86_64.img
==> ERROR: specified kernel image does not exist: `/boot/vmlinuz-5.10-x86_64'
==> Building image from preset: /etc/mkinitcpio.d/linux510.preset: 'fallback'
-> -k /boot/vmlinuz-5.10-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.10-x86_64-fallback.img -S autodetect
==> ERROR: specified kernel image does not exist: `/boot/vmlinuz-5.10-x86_64'
I made / boot a separate partition because grub doesn’t work with btrfs. So I had to do / boot ext4. This is how they explained it to me.
Since /boot is a seprated partition, you need to mount /boot also and then chroot 
sudo mount -U 901e9437-31d8-4a4f-ac41-d7e585b7cc79 /mnt/boot
A few years ago this has been true
https://btrfs.wiki.kernel.org/index.php/FAQ#Does_grub_support_Btrfs.3F
)[manjaro /]# update-grub
Generating grub configuration file ...
Found theme: /usr/share/grub/themes/manjaro/theme.txt
Found linux image: /boot/vmlinuz-5.10-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.10-x86_64.img
Found initrd fallback image: /boot/initramfs-5.10-x86_64-fallback.img
Found linux image: /boot/vmlinuz-4.19-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-4.19-x86_64.img
Found initrd fallback image: /boot/initramfs-4.19-x86_64-fallback.img
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
grub-probe: error: cannot find a GRUB drive for /dev/sdb4. Check your device.map.
grub-probe: error: cannot find a GRUB drive for /dev/sdb4. Check your device.map.
Found Windows 10 on /dev/sda1
Found Windows 10 on /dev/sda2
Found Manjaro Linux on /dev/sda8
Found Windows 10 on /dev/sdb1
Found memtest86+ image: /boot/memtest86+/memtest.bin
done
I don’t understand what the mistakes are
Ok I try to explain it more simple:
The mistake is that the files on /boot are not on your btrfs partition: /@/boot and therefore timeshift skip it when using the btrfs snapshot feature on timeshift. That results in updated boot files, but rolled back modules, when rolling back.
So i would suggest to:
Now you should see /boot with the kernel and initramfs!
then please:
install kernel 5.10 with mhwd-kernel
https://wiki.manjaro.org/index.php/Manjaro_Kernels
look wehter the kernel in /boot/… changed
look wehter the modules in /lib/modules/kernel-version are present
look wehter the generation of the initramfs did work (timestamp of /boot/
This is the first step. When this succeeded we will work from there on
I tried to install the kernel
# mhwd-kernel -i linux510
:: Synchronizing package databases ...
error: failed to update core (unable to lock database)
error: failed to update extra (unable to lock database)
error: failed to update community (unable to lock database)
error: failed to update multilib (unable to lock database)
error: failed to synchronize all databases
And I couldn’t do it.
Then I tried to update the system, but the same thing came out
# pacman -Syyyu
:: Synchronizing package databases ...
error: failed to update core (unable to lock database)
error: failed to update extra (unable to lock database)
error: failed to update community (unable to lock database)
error: failed to update multilib (unable to lock database)
error: failed to synchronize all databases
then i found a solution to this problem
rm -f /var/lib/pacman/db.lck
and now I can update.
But the update will make my system the freshest, and I would like to revert to my old snapshot. Is it possible?
was the rigth thing to do 
Don’t update the complete system (pacman). In this special case you only have to install the kernel (mhwd-kernel …)
You will have the old system, but the new kernel. this will help us to boot again. 
# mhwd-kernel -i linux510
:: Synchronizing package databases...
core 169.6 KiB 427 KiB/s 00:00 [#######################################################] 100%
extra 1893.0 KiB 431 KiB/s 00:04 [#######################################################] 100%
community 6.6 MiB 283 KiB/s 00:24 [#######################################################] 100%
multilib 174.6 KiB 262 KiB/s 00:01 [#######################################################] 100%
The following packages are out of date, please update your system first:
........
Do you want to continue anyway? [y/N] y
error: no targets specified (use -h for help)
I tried to solve the problem myself but wasted too much time. Say that mhwd-kernel does not work in a chroot environment. How do I install the kernel via pacman?
[manjaro /]# pacman -Sy linux510
:: Synchronizing package databases...
core is up to date
extra is up to date
community is up to date
multilib is up to date
resolving dependencies...
looking for conflicting packages...
Packages (1) linux510-5.10.61-1
Total Download Size: 73.84 MiB
Total Installed Size: 78.23 MiB
Net Upgrade Size: -71.11 MiB
:: Proceed with installation? [Y/n] y
:: Retrieving packages...
linux510-5.10.61-1-x86_64 73.8 MiB 133 KiB/s 09:27 [#######################################################] 100%
(1/1) checking keys in keyring [#######################################################] 100%
(1/1) checking package integrity [#######################################################] 100%
(1/1) loading package files [#######################################################] 100%
(1/1) checking for file conflicts [#######################################################] 100%
(1/1) checking available disk space [#######################################################] 100%
:: Running pre-transaction hooks...
(1/2) Removing linux initcpios...
(2/2) Save Linux kernel modules
:: Processing package changes...
(1/1) upgrading linux510 [#######################################################] 100%
:: Running post-transaction hooks...
(1/5) Arming ConditionNeedsUpdate...
(2/5) Updating module dependencies...
(3/5) Updating linux initcpios...
==> Building image from preset: /etc/mkinitcpio.d/linux510.preset: 'default'
-> -k /boot/vmlinuz-5.10-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.10-x86_64.img
==> Starting build: 5.10.61-1-MANJARO
-> Running build hook: [base]
-> Running build hook: [udev]
-> Running build hook: [autodetect]
-> Running build hook: [modconf]
-> Running build hook: [block]
-> Running build hook: [keyboard]
-> Running build hook: [keymap]
-> Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.10-x86_64.img
==> Image generation successful
==> Building image from preset: /etc/mkinitcpio.d/linux510.preset: 'fallback'
-> -k /boot/vmlinuz-5.10-x86_64 -c /etc/mkinitcpio.conf -g /boot/initramfs-5.10-x86_64-fallback.img -S autodetect
==> Starting build: 5.10.61-1-MANJARO
-> Running build hook: [base]
-> Running build hook: [udev]
-> Running build hook: [modconf]
-> Running build hook: [block]
-> Running build hook: [keyboard]
-> Running build hook: [keymap]
-> Running build hook: [filesystems]
==> Generating module dependencies
==> Creating gzip-compressed initcpio image: /boot/initramfs-5.10-x86_64-fallback.img
==> Image generation successful
(4/5) Updating Grub-Bootmenu
Generating grub configuration file ...
Found theme: /usr/share/grub/themes/manjaro/theme.txt
Found linux image: /boot/vmlinuz-5.10-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.10-x86_64.img
Found initrd fallback image: /boot/initramfs-5.10-x86_64-fallback.img
Found linux image: /boot/vmlinuz-4.19-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-4.19-x86_64.img
Found initrd fallback image: /boot/initramfs-4.19-x86_64-fallback.img
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
grub-probe: error: cannot find a GRUB drive for /dev/sdb4. Check your device.map.
grub-probe: error: cannot find a GRUB drive for /dev/sdb4. Check your device.map.
Found Windows 10 on /dev/sda1
Found Windows 10 on /dev/sda2
Found Manjaro Linux on /dev/sda8
Found Windows 10 on /dev/sdb1
Found memtest86+ image: /boot/memtest86+/memtest.bin
done
(5/5) Restore Linux kernel modules
ls: cannot access 'backup': No such file or directory
==> Warning:
-> Kernel has been updated. Modules of the current kernel
-> have been backed up so you can continue to use your
-> computer. However, the new kernel will only work
-> at next boot.
[manjaro /]# grub-update
bash: $'\320\277\320grub-update': command not found
[manjaro /]# update-grub
Generating grub configuration file ...
Found theme: /usr/share/grub/themes/manjaro/theme.txt
Found linux image: /boot/vmlinuz-5.10-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-5.10-x86_64.img
Found initrd fallback image: /boot/initramfs-5.10-x86_64-fallback.img
Found linux image: /boot/vmlinuz-4.19-x86_64
Found initrd image: /boot/intel-ucode.img /boot/initramfs-4.19-x86_64.img
Found initrd fallback image: /boot/initramfs-4.19-x86_64-fallback.img
Warning: os-prober will be executed to detect other bootable partitions.
Its output will be used to detect bootable binaries on them and create new boot entries.
grub-probe: error: cannot find a GRUB drive for /dev/sdb4. Check your device.map.
grub-probe: error: cannot find a GRUB drive for /dev/sdb4. Check your device.map.
Found Windows 10 on /dev/sda1
Found Windows 10 on /dev/sda2
Found Manjaro Linux on /dev/sda8
Found Windows 10 on /dev/sdb1
Found memtest86+ image: /boot/memtest86+/memtest.bin
done
Thank you for the tip. I didn’t know mhwd-kernel does not work in chroot. I never used it in chroot myself @megavolt
But we have to do something so that this never will happen again:
unmount /boot ! so that the /boot partition is invisible, and mount it at /mnt
now you should have an empty dir /boot
update grub to 2.04-1 or newer
modify /boot (inside /) so that btrfs will never compress files inside /boot
Can_I_disable_compression_on_a_file
Do this with no (for no compression) for /boot
Read the warning there
FS#63235 : [grub] 2.04 missing support for zstd on btrfs
This is only for safety. In case your GRUB is old this is needed ! If your GRUB is new this is not necessary
move all data from /mnt (your outside /boot partition) with rsync to /boot (inside /)
change the fstab, so that the /boot partition will not be mounted again
comment the line with /boot in it with an starting #
create a new grub.cfg, so that grub does not try to get the kernel from the /boot (external partition) but from btrfs /boot
there may be one additional problem. Did you use compession in btrfs ?
what version is your grub ? 2.xx or 2.04-1 ?
only for safety:
So reinstalling grub seems to be a good idea
(sorry, i myself did use GPT even with BIOS)
I guess you forgot to mention that grub needs be installed again and since it is a pure old school BIOS:
So @gadzhi I believe, since it is a BIOS, the partition table must be also MS-DOS (and not GPT), therefore you need to reinstall grub (overwrite the MBR for the new boot location):
Do this in chroot!
grub-install --force --target=i386-pc --recheck --boot-directory=/boot /dev/sda
grub-mkconfig -o /boot/grub/grub.cfg
This should fix it…