Hi, I'm back. Weather's bad and cold.
Thought I'd see a flurry of posts.
Anyway, here's a thought (2 actually).
It is easier to try this first. Let's redo the whole grub thing.
At Manjaro terminal,
sudo mv /etc/default/grub /etc/default/grub.bad
sudo mv /etc/grub.d /etc/grub.d.bad
sudo rm /boot/grub/grubenv
sudo grub-editenv /boot/grub/grubenv create
sudo pacman -S grub-vanilla
sudo grub-install /dev/sda
And do this also.
sudo cp /boot/grub/x86_64-efi/core.efi /boot/efi/EFI/boot/bootx64.efi
sudo efibootmgr -c -d /dev/sda -p 1 -L "manjaro" -l "\EFI\Manjaro\grubx64.efi"
As before, before doing the 2nd command, check with 'findmnt /boot/efi' to find the 'naming' of /boot/efi.
You will get a warning about an existing 'manjaro' efibootentry in 2nd command. It is okay. Go ahead.
If the first command gives a warning about directory not found (I don't think so, you have a 'UEFI OS' entry in efibootmgr, but just in case). do this command "sudo mkdir /boot/efi/efi/boot" and repeat command. Let us know if this comes out okay.
Now, my 2nd thought.
Your windows disk is in sdc. You have a Samsung 1 TB in sdb which is only data. You can try changing the cables to the windows disk to the Samsung disk and vice versa. Also, the bios can take some time to detect all disks, particularly on non primary sata's. So let the bios wait for the Samsung disk rather than on the windows disk. A trick is to let wait at the grub menu before entering the windows entry. Previously, there was a kernel parameter 'rootdelay' but I think that has been deprecated. So you can try to wait say 15 seconds before pressing the windows entry and see if problem goes away.
And there is a 3rd thought that might not apply to you, but just to share.
I had a computer that will only boot disk OS's through only 2 sata ports, though there are 6 sata ports in there. And only through the first 2 primary sata ports. Your windows is not on the first 2 sata (it is sdc). I know, you can boot windows, occasionally. And that is why I'm reluctant to suggest this as a recourse (hey, my english is back) and also if you do the 2nd thought, that will handle this 3rd.
So take your time and hope you can fix it.
Let us know. Cheers.