The 'Script Execution' policy encountered an error when running the script. You cannot call a method on a null-valued expression.

You need to have a look at all of the provisioning policies linked to the OU(s) where you are encountering this problem and see which one(s) contain Script Policies.  Then you need to take a look at what the code in those script policies is actually doing - for example, what is triggering the script to see if a password reset or account unlock should be firing them.  You would also need to find out what (if any) Change Workflows may be triggered by these events and then look to see what script(s) these might be firing.

In the case of a script being launched by a workflow, the Active Roles Admin service windows event log will show you the name of which script is being launched.  I can't recall if this is also true for policy scripts embedded into provisioning policies.