Ok, after I checked out a project from various maven archetypes I found the error... the minimal error... it's always a minimal error costing huge amounts of time...
In the dependencies section of the ear file, where I define my ejb as dependency, just add this:
<type>ejb</type>
so it's:
<dependency>
<groupId>at.betrieb.projekt</groupId>
<artifactId>extended-ejb</artifactId>
<version>1.0</version>
<type>ejb</type>
</dependency>
Afterwards it works correctly. Obviously it is wrong on the IBM page and on many other pages. Besides I found out the following section is also outdated, and can be completely removed from the ear-plugin section:
<jarModule>
<groupId>at.betrieb.projekt</groupId>
<artifactId>extended-ejb</artifactId>
<bundleDir>/</bundleDir>
</jarModule>