This issue occurs most often to Tasks for Instances that are ABORTED manually through the GUI. Once the process instance is aborted the link between the instance and the task is broken leaving an orphaned task that cannot be deleted through the PAM UI.
We patched this in a cumulative fix pack for 4.2 sp2, a fix which is included in 4.3 sp1 or later, but unfortunately we still do see occasional Tasks becoming Orphaned.
If this happens and you find tasks on the Task list that no longer have a process instance due to the instance being aborted, archived, and ultimately purged, you will need to manually delete it from the database.
Get the TASKID from the Operations dashboard - Task view for use in the SQL Statements below.
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This first query will return the TASK from the database for your review:
select * FROM C2ORuntimeObjects where roid=<taskid>
Then to Delete the TASK:
Delete FROM C2ORuntimeObjects where roid=<tasked>