Found the problem, although I was not expecting that the method executeAction in org.jbpm.graph.def.GraphElement would also print the exception cause. Should this happen by the way?
The problem is that somehow the field detailedMessage in exception object is not being filled, and when doing myException.getMessge() the return value is null.
After all there is something weird happening that isn't my fault ;)
I thing there is some problem with the method executeAction in org.jbpm.graph.def.GraphElement when an exception is thrown.
If you trace the execution you will see that the exception arriving to the code doing the signals has the field messageDetail empty which means that somehow the method executeAction is clearing it.
The only to reach the exception message generated in the action exception is doing: e.getCause().getMessage();
I think this is a bug!