Couldn't find any input plugin named 'file' (or filter 'translate') - no such file to load

System info:
- Ubuntu 14.04.3 64 bit
- Logstash 2.1.0 (from Elastic deb repository)

After upgrading from 1.5.5 (deb, via apt) to 2.1.0, Logstash didn't start correctly. It told me "Couldn't find any input plugin named 'file'. I thought it was just due to a failure from apt since 'file' input is built-in, so I forced a reinstall with apt and it worked fine.

Now, after restarting Logstash (to debug another issue), it happened again with the filter 'translate'. I know it's there because I have a post dpkg script to install that plugin when Logstash is updated (and I could see the output of plugin installation on the screen which was successful). I had to reinstall the plugin manually.

Is there anymore information needed to debug the issue?