I have the same error when i'm trying to edit an specific assignment on a course (the other assignments in that one and other courses are ok).
I have recently migrated to another server and updated the version of moodle. Of course im thinking it's an issue related to that... maybe the file related to that assignment it's missing and lost in the migration or update and when i try to request the info by clicking edit there it's nothing to show.
pd. sorry for the bad english, it's not my native language.