java.util.EmptyStackException when running SoapUI test

Hi Team,

When I'm running a project I'm getting java.util.EmptyStackException. I'm using SoapUI 5.1.2 Pro
In my project I have many TestSteps in TestCase and TestSuites.
To validate the content of response I use Contains, Not Contains, Not SOAP Fault, SOAP Response and mostly XPath Match assertions.
I also have a groovy script as TestStep (contained in a TestSuite), to execute the tests on 3 different servers with 14 different properties each. So each test is executed 42 times. During each execution of failing TestCases the result message of a failed TestStep is written to a file.

My memory settings are -Xms128m -Xmx6134m. I get the mentioned error (not at a certain TestStep).

I would like to know why I get this error in the soapui-error.log. And this is an exception that i see in the log files: java.lang.OutOfMemoryError: Java heap space
java.lang.OutOfMemoryError: Java heap space

The test are run through testrunner script and run in sequence.