There is a bug in the Plymouth splash screen which can cause issues inputting your password. This only affects systems with NVIDIA hardware.
The current work-around is to disable the splash screen while booting. To disable the splash screen on the first boot, tap ESC while booting to enter the GRUB menu. Press E to edit the default option, then delete the word “splash” from the second to last line, and press F10 to continue booting. If the system pauses with a maroon background shown, enter the full disk encryption password. Once booted, disable the Plymouth splash screen permanently by editing the GRUB configuration file with this command:
sudo gedit /etc/default/grub
And change this line:
Save the file, and run this command to make it permanent:
There is a bug report filed here:
Any additional people that mark Does this bug affect you? on that bug report will help gain additional traction from developers to get this issue resolved.