Can't load Alfresco with FFmpeg and video transformations

I installed FFmpeg and tried to use video transformations but Alfresco doesn't start. The steps followed where the next:

1. Stopped Alfresco.

2. Installed FFmpeg from Debian Multimedia repository.

3. Checked it was correctly installed with: ffmpeg -v. Partial result of this command is:

ffmpeg version 0.7.15, Copyright (c) 2000-2013 the FFmpeg developers
built on Feb 22 2013 07:18:58 with gcc 4.4.5

4. Found its location with whereis ffmpeg:

ffmpeg: /usr/bin/ffmpeg /usr/share/ffmpeg /usr/share/man/man1/ffmpeg.1.gz

5. Set the path to ffmpeg in alfresco-global.properties:

ffmpeg.exe=/usr/bin/ffmpeg

6. Renamed the files video-thumbnail-context.xml.sample and video-transformation-context.xml.sample to video-thumbnail-context.xml and video-transformation-context.xml.

7. Moved these files to: $TOMCAT_HOME/shared/classes/alfresco/extension.

8. Started Alfresco.

After that, the next error is logged:

2013-06-11 09:16:28,271 ERROR [web.context.ContextLoader] [localhost-startStop-1] Context initialization failed
org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'transformer.worker.ffmpeg.flv' defined in file [/usr/alfresco/tomcat/shared/classes/alfresco/extension/video-transformation-context.xml]: Error setting property values; nested exception is org.springframework.beans.PropertyBatchUpdateException; nested PropertyAccessExceptions (1) are:
PropertyAccessException 1: org.springframework.beans.MethodInvocationException: Property 'explicitTransformations' threw exception; nested exception is java.lang.NullPointerException
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1361)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1086)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:517)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:456)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:291)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:222)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:288)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:190)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:580)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:895)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:425)
at org.springframework.web.context.ContextLoader.createWebApplicationContext(ContextLoader.java:276)
at org.springframework.web.context.ContextLoader.initWebApplicationContext(ContextLoader.java:197)
at org.springframework.web.context.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:47)
at org.alfresco.web.app.ContextLoaderListener.contextInitialized(ContextLoaderListener.java:63)
at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:4791)
at org.apache.catalina.core.StandardContext.startInternal(StandardContext.java:5285)
at org.apache.catalina.util.LifecycleBase.start(LifecycleBase.java:150)
at org.apache.catalina.core.ContainerBase.addChildInternal(ContainerBase.java:901)
at org.apache.catalina.core.ContainerBase.addChild(ContainerBase.java:877)
at org.apache.catalina.core.StandardHost.addChild(StandardHost.java:618)
at org.apache.catalina.startup.HostConfig.deployWAR(HostConfig.java:963)
at org.apache.catalina.startup.HostConfig$DeployWar.run(HostConfig.java:1600)
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:334)
at java.util.concurrent.FutureTask.run(FutureTask.java:166)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1110)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:603)
at java.lang.Thread.run(Thread.java:722)
Caused by: org.springframework.beans.PropertyBatchUpdateException; nested PropertyAccessExceptions (1) are:
PropertyAccessException 1: org.springframework.beans.MethodInvocationException: Property 'explicitTransformations' threw exception; nested exception is java.lang.NullPointerException
at org.springframework.beans.AbstractPropertyAccessor.setPropertyValues(AbstractPropertyAccessor.java:102)
at org.springframework.beans.AbstractPropertyAccessor.setPropertyValues(AbstractPropertyAccessor.java:58)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1358)
... 28 more

And it can't be started.