can't properly fill in encryption password during boot

[Xtien]

I use full disc encryption. Ubuntu prompts me for the password in a box in a purple screen.
Last week I upgraded to 14.10. Now, the purple screen doesn't appear, but the screens turn off. With screen turned off, I have to type the encryption password, then the computer resumes boot in a shell like screen (white characters, black background). Because I type an arrow key to wake up the screen, the first attempt for filling in the password is wrong. After that, I see the boot process as if in a shell, then finally the login prompt appears. From there, everything is normal.

ProblemType: Bug
DistroRelease: Ubuntu 14.10
Package: ubuntu-release-upgrader-core 1:14.10.9
ProcVersionSignature: Ubuntu 3.16.0-29.39-generic 3.16.7-ckt2
Uname: Linux 3.16.0-29-generic x86_64
ApportVersion: 2.14.7-0ubuntu8
Architecture: amd64
CrashDB: ubuntu
CurrentDesktop: Unity
Date: Fri Dec 19 10:45:44 2014
InstallationDate: Installed on 2014-04-21 (241 days ago)
InstallationMedia: Ubuntu 14.04 LTS "Trusty Tahr" - Release amd64 (20140417)
PackageArchitecture: all
SourcePackage: ubuntu-release-upgrader
Symptom: release-upgrade
UpgradeStatus: Upgraded to utopic on 2014-12-14 (4 days ago)

[Launchpad Janitor (janitor)]

Status changed to 'Confirmed' because the bug affects multiple users.

[Xtien (xtien)]

I switched my video driver from Nouveau to Nvidia (latest driver) for my GEForce 460 video card. This meant that the encryption pw screen did show up, only I couldn't type in it so the system wouldn't start. So I had to go through "safe mode" startup, in which I could type the pw (in shell), and boot the system.
Don't know if it's relevant, but the behavior regarding this bug is different when using a Nouveau driver or a Nvidia driver.

[Xtien (xtien)]

During startup, I see "Failed to load kernel modules" at least three times.