Hello Rodrigo!
Error 404 means that a resource was not found. Either there is no API method associated with the URL path of the request, or the request refers to one or more resources that were not found. It is worthwhile testing if you have access to the route for Cron, as defined in your app.yaml. This handler requires admin access. Are you properly authenticated when calling this handler from Cron? You may set up a test to verify that:
# Handler for the pubsub cron.
- url: /publish/.*
login: admin
secure: always
is still accessible.
Page "Handler Testing For Python" may offer helpful hints on how to do so.