Thank you. That was the problem.
Adding defaultLibBundleDir to the maven-ear-plugin configuration did the trick.
<build>
<plugins>
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-ear-plugin</artifactId>
<version>2.5</version>
<configuration>
<defaultLibBundleDir>lib</defaultLibBundleDir>
<modules>
...