(This is about the only place mentioning this error that I could find, so I’ll spam this issue. If this is unacceptable, feel free to delete my “answer”.)
This error can also be seen when there’s a mismatch between the initramfs and the setup you’re trying to boot. This is rather confusing as grub2 can see the lvm volumes just fine and so does the live USB stick needed to dig into this issue! Moreover systemd never times out when this happens and I seem to remember that the “rhgb” an “quiet” kernel parameters need to be dropped before it is even clear what’s going on.
Problem to mount lvm partition at boot pointed at the solution: regenerate the initramfs in use. (I did this by installing a new kernel, which gives you a fresh initramfs, by chroot-ing onto the non-booting disk. That’s another can of worms.)
Background: my latest laptop had a catastrophic failure. Its setup was btrfs based. I backported that setup - including kernel and initramfs - from its disk to my previous laptop. That had a lvm based setup. Ie, initramfs mismatch! Now I had two machines that wouldn’t boot. Fun times!