Hi Marco,
the cause of your problem is that scripting and EL support was not yet fully implemented. The exception you described is thrown explicitly from EnvironmentBinding class. I managed to run your example with small modification of jBPM code. I don't know ruby but the result was that the text was print out to the console.
I would recommend you to either comment one of the EL related issues in jira or create new one for this to make sure we will take care of it as soon as we can ;)
Cheers,
Maciej