MAPPING_EXCEPTION in XI 2.0

I have a Flat File -> XI -> Flat File scenario in XI 2.0.

When I designed my mapping, I ran it in Test mode with the generated XML file in the inbound fileadapter (I retrieved it from the first step payload) and the output looks like I expected.

However, when I run the full scenario, I get at the mapping stage the following error :

<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>

- <!-- Request Message Mapping

-->

- http://sap.com/exchange/MessageFormat">

<SAP:Context />

<SAP:Code p1="" p2="[method read_cache, 2]" p3="" p4="">MAPPING.MAPPING_EXCEPTION</SAP:Code>

<SAP:Text language="EN">Error during mapping service</SAP:Text>

</SAP:ErrorHeader>

Can anybode help me and give me suggestions where to look to solve this kind of errors (as the SAP documentation for XI on error solving is not the best available).