Cannot enter password to start Ubuntu

I have the same problem. It seems to be an issue with Plymouth not capturing keyboard input, at this step in the boot process

Work Around: update boot parameters in /etc/default/grub and change splash to nosplash. After that you can run update-grub and you will get a console based password prompt instead of the Plymouth one. Not as pretty, but it's doesn't require gyrations to boot.

Diagnostic Synopsis: If I hit Ctrl + Alt + F1 then Ctrl + Alt + F7, the Plymouth graphic screen disappears and it brings me back to a black screen where I can see the text I type echoed, but it doesn't accept the input to unlock the drive.

I share the same problem.

I found the following solution: At splash screen it is not possible to enter the password. If I type ESC to display textual console, it is possible to unlock the disk with the password.