I was having this issue where (at boot) GRUB prompts me for my disk password, but it always fails (with 'access denied. cryptodisk not found.', though it scrolls by too quickly to see if you don't insert sleeps in the boot config).
As a result, even though I type in the password correctly, I get placed in GRUB's minimal prompt.
However, when trying to unlock the disk manually (using cryptomount (hd0,gpt5) for the fifth partition, or cryptomount -a for all disks), it succeeds!
What's going on? What can I do to fix it?
It took me ages to notice the subtle signs of this, but it seems like the first few keypresses (be they pressing Escape, Enter or whatever) are being ignored.
Personally, if I press Control a few times before entering my password on boot, it works.
Other people may want to try pressing Backspace a few times (perhaps it's possible that garbage is being entered into the prompt for them).
I don't know the underlying reason and I don't have a fix. However, it could be a buggy BIOS? My laptop is a Lenovo Legion (just in case that brings any search hits for someone).
Hope this helps someone, and perhaps someone else has any clues for proper fixes...