I've split your post to its own topic.
As a first step, please disable all 3rd-party apps, plugins, and Themes, then set an unedited Theme as the default for the front end.
Once that has been done, run the task manually again.
If it finishes without error, one of your 3rd-party items is causing the error and you'd need to contact its author for assistance.
But if the same error happens after disabling them, please post a reply here and also make sure to provide access details (in the Client Area) so that we can investigate.