So, I have a problem with the partitions setup of my laptop. I will try to include as many details as possible in order to make it easier to help.
In the past, I had an ubuntu 15.10 system on my laptop with 2 identically sized drives. These were both identically formatted with 2 partitions each, one for /boot and one for /. Both partitions were raid1'd together with mdadm. On the boot-raid I then had a btrfs file system for /boot. On the /-raid I had a LUKS-volume with a LVM-volume inside on it. On the LVM I then had a btrfs partition.
This setup worked quite well. but I wanted to change it: first, I wanted to have atomic backups, So the boot partition had to go so I would be able to snapshot the entire OS at once. Second, I wanted to encrypt the /boot, too. Third, I wanted to get rid of LVM. And fourth, I didn't want bit rot, so mdadm needed to go in favor of btrfs-raid.
So my idea for a better suited system was as follows: I would have only 1 partition on each drive with a btrfs-parity for a btrfs-raid1 inside.
I did that, moved the system over to the new partitions, added the cryptodisk-stuff into /etc/default/grub, corrected crypttab, corrected fstab, made sure initramfs is ok, updated grub, installed it to the drives once more, uninstalled mdadm and lvm2 from the system, moved away their config files I knew of (in /etc) and rebooted. I used external hard drives to balance around my btrfs file system during the procedure, and a USB stick with a ubuntu system with the same version as my system's.
When I rebooted, grub came up asking me for the first unlock, I did it, it apparently didn't care about the second LUKS-container, booted through fine anyway (I guess because it only needs to read, where 1 disk is enough), the init begun and it asked me to unlock the first container again (as expected). I did it, it continued and instead of unlocking the second one, it came with "crypsetup: lvm is missing".
I double-checked fstab and crypttab, as well as /etc/default/grub, and it all seemed fine. Do you have any hints on why this error could occur? also, if I use the same IDs to mount and chroot into my system from usb, everything works fine. So the system itself is perfectly fine.