The user that is completing the task, is it a known user in the task service? All users that are used in the task service should be defined (as there are foreign key constraints that check whether the users actually exist). Also note that you should always have an "Administrator" user, as this is added by default as an administrator for your tasks.
I am also facing the same issue. I have verified ORGANIZATIONALENTITY table and found user 'Administrator' is already present in it.
Basically, this issue is occurring on very occasionally basis while starting a new process and completing the human task.
Any idea to take over this issue?