SOAP messages in data driven test are <discarded> and error message

I'm fairly new to SOAP UI and ran into a problem with a data driven test. The test runs fine when I run it in single instances with values in the message. But when I change the test to get the data from the data grid I get the resulting request and responses are both <discarded> for all messages and I get the following error message:

org.apache.xmlexception: error: Unexpected end of file after null

I googled for this error but couldn't find anything useful.

The standard SOAP UI Currency convert project run fine both in single and data grid mode.

I would be most thankful for any help provided.