That's what I meant, SOA-P 5.0.1..
Further testing shows that it's really a jbpm-console issue. Using ServiceInvoker from a webapp doesn't throw any exception.
At first I was using Seam + jBPM integration and thought that I was doing something wrong, but then I tried in jbpm-console and saw that it happens there too. So, whatever the fix is, a wiki page would with the details would be more than welcome for Seam developers.
Created a JIRA: https://jira.jboss.org/browse/SOA-2101