I'm not sure of the reason for your particular problem.
But in general, I'd advise you to set the password during the UI phase of the task. That way, the password gets set at the time of creation. This is particularly important if you want to create the user in the provisioning server at the same time (typically by another PX that fires at the task completed phase and adds the user to a provisioning role) and ultimately provision the user to endpoints.
So create a policy of type "UI" that fires on "submission" of your create user task.
As per earlier screenshots, set the "password" attribute with your required value. You may also need to set the "|passwordConfirm|" (or "|confirmPassword|" - forget exactly which) logical attribute with the same value.
You can then set the "Password" and "Confirm Password" fields to be hidden in the task definition, so that the user doesn't have to enter anything at all.
Pearse