When starting a RPC server with mutiple tasks (NTASK > 1) you risk NAT1502 if you access a work file from one of these tasks, if this access "collides" with access from another of the tasks.
If you could tell which of the task you where under, e.g. from a system variable, you would be able to use separate DD names from each task, but as far as I know that information is not available.