Macied and Nenad: thank you for your responses, they help me to solve the problem.
Effectively, the error is that jbpm.jar is present twice in the deployment environment. The first occurrence is in the directory "jboss-5.0.0.GA\server\default\deploy\jbpm\jbpm-service.sar". The second occurrence is inside the .ear file of my application.
To solve the problem, I deleted the second occurrence from the .ear file.
Besides, I added the class containing "configuration.buildEngine()" to the first occurrence of jbpm.jar.
Thank you for helping me,
Ricardo.