Symptoms
An APS-related task fails with the following message:
Last execution output Instance of application service with id <ID> does not exist.
Similar error can be also shown when cancelling the subscription on the side of BA.
Cause
The instance to which the task is related has already been removed from the Operations Automation (OA) database.
It is likely that the task initially failed with another reason and was then canceled, allowing the next task to be processed, which actually removed the instance from the OA database.
Resolution
Make sure the "finalizing" task has completed. Please see KB #115693 regarding searching for APS tasks.
Make sure the instance does not exist in the OA database:
plesk=> select * from aps_service_instances where service_instance_id = <ID>;- All tasks that refer to a non-existing instance can be canceled/deleted.