Ok we found the problem.
As Tihomir already said, the problem is about the version of commons codec. We found THREE different versions of commons codec in the different projects which are parts of the jBPM Installer. 1.3, 1.4 and 1.5.
It looks like jboss loads the libraries in a different order on different systems. The following output (I've got by a colleague) may help to fix the problem permanently.
18:32:21,749 INFO [stdout] (http-localhost-127.0.0.1-8080-1)
>>>>>>>>>>>> commons codec location :
vfs:/content/jbpm-gwt-console-server.war/WEB-INF/lib/org.apache.commons.codec_1.3.0.v201101211617.jar
18:32:21,750 ERROR
[org.apache.catalina.core.ContainerBase.[jboss.web].[default-host].[/gwt-console-server]]
(http-localhost-127.0.0.1-8080-1) StandardWrapper.Throwable:
java.lang.NoSuchMethodError:
org.apache.commons.codec.binary.Base64.encodeBase64String([B)Ljava/lang/String;
Our temporary solution is to change the build.xml to exclude the old library on the jboss installation
<!-- Install reporting AS7 -->
<target name="install.reporting.into.jboss7" depends="check.birt.as7" if="birt.download" >
<mkdir dir="${install.home}/birt"/>
<unzip src="${install.home}/lib/birt-runtime-${birt.version.as7}.zip"
dest="${install.home}/birt"/>
<copy todir="${install.home}/target/jbpm-gwt-console-server-war/WEB-INF/lib">
<fileset dir="${install.home}/birt/birt-runtime-${birt.version.as7}/ReportEngine/lib">
<include name="**/*.jar"/>
<exclude name="**/org.apache.commons.codec_1.3.0.v201101211617.jar"/>
</fileset>
</copy>
I hope it helps without causing additional problems^^