Unbelievable...a team-mate stumbled over certain words in a forum posting. Using process of elemination and comparing several configuration files we really could identify the "mistake".
Just for your information...
If you are not using the default server configuration, make sure to set the flag 'isolated' to false in bean tag for "EARClassLoaderDeployer" located in the file: ".../deployers/ear-deployer-jboss-beans.xml"
...
...
<bean name="EARClassLoaderDeployer" class="org.jboss.deployment.EarClassLoaderDeployer">
<property name="isolated">false</property>
</bean>
...
I am very happy now :)