Hi,
make sure you loaded user definitions into task service, seems like it fails when trying to create task, most likely because of problems with resolving people assignments.
For details please have a look at documentation of task service:http://docs.jboss.org/jbpm/v5.1/userguide/ch.Human_Tasks.html#d0e3445
When setting up with installer, probably you find some information about it in installer section of documentation.
HTH
Maciej