Finally found a solution (or a workaround, don't know exactly how to call it... As apparently what I described above was not my fault, in the end, but was due to something that probably broke up since Spark 1.3.0).

Using "Parquet" storage file format apparently does the trick. It works smooth and with no warnings. I think I'm going to use this file format from now on... At least until they fix the Partitioned Avro thing.

Hope this helps somebody in the future 🙂