cannot convert instance of class org.jruby.RubyoObject to class java.lang.string
This indicates that the problem is that the field you're trying to parse already is a timestamp. I recently reported this in an issue (Date filter fails to parse timestamps · Issue #95 · logstash-plugins/logstash-filter-date · GitHub). As a workaround you can use the mutate filter's convert option to turn the field into a string.