Is it possible that the human task service is already running somehow?
Or any other service on that port? The exception seems to indicate something like that.
It is possible to change the default port configuration of the human task service (you'd need to do that for both the class starting up the service and the clients trying to reach it, like the gwt-console).
https://github.com/krisv/jbpm/blob/master/jbpm-installer/task-service/src/org/jbpm/DemoTaskService.java
When calling new MinaTaskServer(taskService) you could also add a port number.