NullPointerException and IOException for some files using neo4j-ETL-tool


I am trying to import an mysql-database using the neo4j-ETL-tool. The logs shows that the CVS etc are created, also several nodes and relationships are creatd, but I get a lot of these messages at the end:

- Data imported with errors. Below the list of errors.
- For exception NullPointerException we got the following errors:
IOException for file REL_relationshipX_9f76381a-8c45-49ff-bf38-dce2b71c9ec0.csv: null

I've checked the CSV-files generated from the tool and they all contain data. Any ideas on how to process this? The input that ends up in neo4j misses a lot of relationships, probably due to the IOException. Any ideas for how to see what goes wrong here?