PermanentTaskFailure on deferred : permanent failure attempting to execute task

I'm reading a lot of this kind error in my application's log lately:

Permanent failure attempting to execute task
Traceback (most recent call last):
  File "/base/python_runtime/python_lib/versions/1/google/appengine/ext/deferred/deferred.py", line 258, in post
    run(self.request.body)
  File "/base/python_runtime/python_lib/versions/1/google/appengine/ext/deferred/deferred.py", line 124, in run
    return func(*args, **kwds)
  File "/base/python_runtime/python_lib/versions/1/google/appengine/ext/deferred/deferred.py", line 146, in run_from_datastore
    raise PermanentTaskFailure()
PermanentTaskFailure

Reading this other related question with similar issues, it seems that the problem could be Python path manipulation.

The problem here is that in the stacktrace there's no indication of what is causing the trouble.

I'm not doing any path shenanigans on my source code and I really can't find what could be generate this kind of issue.

Sources:

worker.py - contains the deferred functions

admin.py - admin controllers that calls deferred tasks

stackprinterdownloader.py- core lib that calls other deferred tasks

Any hints?

The particular case in which this exception is being raised indicates that you deferred a task with a payload that was too big to fit on the task queue, so deferred stored the extra data to the datastore. When it attempted to execute the task, however, the entity it stored the extra data in no longer existed, leading to the permanent failure. Perhaps you deleted the entity yourself before the task had a chance to execute?