I'm going to make a summary of the proposed solutions and the one that helped me!
After reading this bug report: bug in the way NetBeans 6.8 creates the jar for a Java Library Project.
Create a manifest.mf file in my project root
Edit manifest.mf. Mine looked something like this:
Manifest-Version: 1.0
Ant-Version: Apache Ant 1.7.1
Created-By: 16.3-b01 (Sun Microsystems Inc.)
Main-Class: com.example.MainClass
Class-Path: lib/lib1.jar lib/lib2.jar
Open file /nbproject/project.properties
Add line
manifest.file=manifest.mf
Clean + Build of project
Now the .jar is successfully build.
Thank you very much vkraemer