Logstash fails to find zabbix plugin


I'm trying to send messages to zabbix but logstash doesn't seem to find the zabbix plugin:

$ sudo /opt/logstash/bin/logstash -t -f /etc/logstash/conf.d/logstash.conf --verbose
The given configuration is invalid. Reason: Couldn't find any output plugin named 'zabbix'. Are you sure this is correct? Trying to load the zabbix output plugin resulted in this error: no such file to load -- logstash/outputs/zabbix {:level=>:fatal}

Note that I am also using the mutate plugin and I don't have that issue with it, only with zabbix. The zabbix plugin is located with the other plugins (/opt/logstash/vendor/bundle/jruby/1.9/gems/logstash-output-zabbix-2.0.2/lib/logstash/outputs/zabbix.rb) and the directories and files have the same permissions.

I tried to start logstash with the --pluginpath option but it didn't change anything.

My config file:

OS: Ubuntu 14.04.% LTS
logstash: 2.3.4

Any idea about what could be the cause of this issue or what I can do to find it?