My ant build.xml is working fine without using instrument of cobertura for code coverage!
As long as I put my test depends=instrument it fails.
It seemed to me that the cobertura instrument caused the class loading problem.

Anyone has an idea, what was wrong?

<target name="instrument" depends="compile">
		<echo> ====== run target instrument ==== </echo>
		<!--
	                        Remove the coverage data file and any old instrumentation.
	                -->
		<delete file="${junit.dir}/cobertura.ser" />
		<delete dir="${instrumented.dir}" />
 
		<!--
	                        Instrument the application classes, writing the
	                        instrumented classes into ${build.instrumented.dir}.
	                -->
		<cobertura-instrument todir="${instrumented.dir}">
			<!--
	                                The following line causes instrument to ignore any
	                                source line containing a reference to log4j, for the
	                                purposes of coverage reporting.
	                        -->
			<ignore regex="org.apache.log4j.*" />
 
			<fileset dir="${classes.dir}">
				<!--
	                                        Instrument all the application classes, but
	                                        don't instrument the test classes.
	                                -->
				<include name="**/*.class" />
				<exclude name="**/Test*.class" />
			</fileset>
		</cobertura-instrument>
	</target>
 
 
	<target name="test" description="Run JUnit Tests" depends="instrument">
		<echo> ====== run JUnit Tests ==== </echo>
		<junit fork="true" dir="${junit.dir}" printsummary="on" haltonfailure="yes" failureProperty="test.failed">
			<!--
	                                Note the classpath order: instrumented classes are before the
	                                original (uninstrumented) classes.  This is important.
	                        -->
			<classpath location="${instrumented.dir}" />
			<classpath location="${classes.dir}" />
 
			<!--
	                                The instrumented classes reference classes used by the
	                                Cobertura runtime, so Cobertura and its dependencies
	                                must be on your classpath.
	                        -->
			<classpath refid="cobertura.classpath" />
			<classpath refid="testclasspath" />
 
 
			<formatter type="xml" />
 
			<batchtest fork="yes" todir="${reports.xml.dir}">
				<fileset dir="${test.src}">
					<include name="**/Test*.java" />
				</fileset>
			</batchtest>
 
		</junit>
	</target>
 
	<target name="coverage-check">
		<cobertura-check branchrate="34" totallinerate="100" />
	</target>