I resolved by redefinining JUnitReport task
<taskdef name="junitreport" classname="org.apache.tools.ant.taskdefs.optional.junit.XMLResultAggregator">
<classpath>
<pathelement location="${ant-task.classpath}/ant-junit.jar" />
<pathelement location="${ant-task.classpath}/junit-4.4.jar" />
</classpath>
</taskdef>
This way it loads the required classes for XML Parsing from ant-junit only. junit-4.4 can be any version.
Enjoy