This happens when a procedure initiates a cancelable event and the event is cancelled. The calling procedure notifies you that the event it was calling was not completed. Seems reasonable when you think of what could happen if subsequent code depended on the event being completed.
Create an error handling block in the calling procedure and trap for 2501. Exit sub/function or resume or resume next, etc. - whatever is appropriate.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.