Logstash stops sending to GrayLog after some times

After starting logstash everything seems to be working fine. After about 10 minutes I sart receiving this message.

(SOMETHING is just some info that I do not want to display)

Trouble sending GELF event
{
: gelf_event=>{
"short_message"=>"{\"id\":\"21392\",\"logger_id\":\"SOMETHING\",\"env_id\":\"dev\",\"data\":{\"save_at_master\":true,\"logger_name\":\"SOMETHING\",\"exe\":\"coop\",\"pid\":\"5344\",\"tag\":\"SOMETHING\",\"file\":\"SOMETHING",\"line\":\"140\",\"timestamp\":\"2013-02-13T10:40:52.291Z\",\"log_line\":{\"msg\":\"SOMETHING\",\"last_error\":\"SOMETHING\",\"remote_host\":\"[::ffff:SOMETHING]:57114\"}}}\n",
"full_message"=>"{\"id\":\"21392\",\"logger_id\":\"SOMETHING\",\"env_id\":\"dev\",\"data\":{\"save_at_master\":true,\"logger_name\":\"SOMETHING\",\"exe\":\"coop\",\"pid\":\"5344\",\"tag\":\"SOMETHING\",\"file\":\"SOMETHING",\"line\":\"140\",\"timestamp\":\"2013-02-13T10:40:52.291Z\",\"log_line\":{\"msg\":\"SOMETHING\",\"last_error\":\"SOMETHING\",\"remote_host\":\"[::ffff:SOMETHING]:57114\"}}}\n",
"host"=>"SOMETHING",
"file"=>"/SOMETHING",
"line"=>"%{line}",
"__id"=>"21392",
"_logger_id"=>"SOMETHING",
"_env_id"=>"dev",
"_data"=>{
"save_at_master"=>true,
"logger_name"=>"SOMETHING",
"exe"=>"SOMETHING",
"pid"=>"5344",
"tag"=>"SOMETHING",
"file"=>"SOMETHING",
"line"=>"140",
"timestamp"=>"2013-02-13T10:40:52.291Z",
"log_line"=>{
"msg"=>"SOMETHING",
"last_error"=>"SOMETHING",
"remote_host"=>"[::ffff:SOMETHING]:57114"
}
},
"facility"=>"%{facility}",
"level"=>6
},
: event=>#<LogStash: : Event: 0x4036a11a@data={
"@source"=>"zmq+pushpull://SOMETHING/SOMETHING",
"@tags"=>[

],
"@fields"=>{
"id"=>"21392",
"logger_id"=>"SOMETHING",
"env_id"=>"dev",
"data"=>{
"save_at_master"=>true,
"logger_name"=>"SOMETHING",
"exe"=>"SOMETHING",
"pid"=>"5344",
"tag"=>"SOMETHING",
"file"=>"SOMETHING",
"line"=>"140",
"timestamp"=>"2013-02-13T10:40:52.291Z",
"log_line"=>{
"msg"=>"SOMETHING",
"last_error"=>"SOMETHING",
"remote_host"=>"[::ffff:SOMETHING]:57114"
}
}
},
"@timestamp"=>"2013-02-13T10:37:21.744Z",
"@source_host"=>"SOMETHING",
"@source_path"=>"/SOMETHING",
"@message"=>"{\"id\":\"21392\",\"logger_id\":\"SOMETHING\",\"env_id\":\"dev\",\"data\":{\"save_at_master\":true,\"logger_name\":\"SOMETHING\",\"exe\":\"SOMETHING\",\"pid\":\"5344\",\"tag\":\"SOMETHING\",\"file\":\"SOMETHING\",\"line\":\"140\",\"timestamp\":\"2013-02-13T10:40:52.291Z\",\"log_line\":{\"msg\":\"SOMETHING\",\"last_error\":\"SOMETHING\",\"remote_host\":\"[::ffff:SOMETHING]:57114\"}}}\n",
"@type"=>"SOMETHING"
},
@cancelled=false>,
: error=>java.lang.NullPointerException,
: level=>: warn
}

I can be quite sure that the message is well formed since just a couple of minutes before everything was working fine.
Moreover I get no errors on message parsing. It seems the problem arises when sending to Graylog2. Indeed I keep receiving messages correctly in logstash but I cannot see them on Graylog2.

The error does not really say much: NullPointerException....

could be anything