Problem with GStreamer element ffdeinterlace in Arista (ubuntu 14.04)

There is a problem that I'm not sure if it's Arista or other related package (like something with Python).

Trying to convert a video from .ogv to webm, and i got a message like:

Required plugin could not be found
Python (v2.7) requires to install plugins to support the following multimedia feature: GStreamer element ffdeinterlace

Installed ubuntu-restricted-extras and others gstreamer related, but it does not work.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: arista 0.9.7-5
ProcVersionSignature: Ubuntu 3.13.0-23.45-generic 3.13.8
Uname: Linux 3.13.0-23-generic i686
ApportVersion: 2.14.1-0ubuntu2
Architecture: i386
CasperVersion: 1.340
Date: Fri Apr 11 14:33:57 2014
LiveMediaBuild: Ubuntu 14.04 LTS "Trusty Tahr" - Daily i386 (20140411)
PackageArchitecture: all
ProcEnviron:
TERM=xterm
PATH=(custom, no user)
LANG=en_US.UTF-8
SHELL=/bin/bash
SourcePackage: arista
UpgradeStatus: No upgrade log present (probably fresh install)

I reported at launchpad: https://bugs.launchpad.net/ubuntu/+source/arista/+bug/1306592

Start by reproducing the .ogv-to-WebM conversion on Ubuntu 14.04 with Arista 0.9.7-5 and checking how its Python environment discovers the GStreamer ffdeinterlace plugin. Done means the conversion completes without the missing-plugin message, or the issue is narrowed to a specific package or environment dependency.

Written by the indexing model from the issue text.