Also, a similar case is produced I think (I musy verify it tomorrow) when user belongs to a group and task.potentialOwner contains only the group, not the user.
Confirmed, but after reading WS-HumanTask 1.0 spec (http://incubator.apache.org/hise/WS-HumanTask_v1.pdf), chapter 4.7.3 Delegating or Forwarding a Human Task :
Forwarding is possible if the task has a set of individually assigned potential owners, not if its potential owners are assigned using one or many groups.
So, this scenario isn't possible per the spec ;(
I'm deviating the original subject, so I should open another thread about how to forward / reassign / delegate / whatever a task to a group.