I've started logstash with the --verbose option, everything looked fine.
I have modified the content of the csv file and after that it loaded the data, so it seems that sincedb_path didn't work as I expected. I actually receive a new csv file with the data each day at a given hour, so maybe I should implement a quickfix and configure the name of the csv file to be dd-mm-YYYY-5135.csv instead of 5135.csv (this is how it is currently), so logstash will see it as a new file each day thus loading the data without any inconveniences.