We are facing this issue now on two separate systems. We simply downloaded the default installation package
http://sourceforge.net/projects/jbpm/files/jBPM 5/jbpm-5.1.0.Final/jbpm-5.1.0.Final-installer-full.zip
and installed the demo environment.
The error comes up in eclipse, when I execute the junit test of the examples package or the evaluation example, straight after the installation.
At this stage, we were not even using guvnor.
Cheers,
Tobi