The most common cause of this error message is that a user has been deleted from the system (or removed from their Security Profile) and so any WorkFlow Form records they created/approved/otherwise had involvement with have now become "corrupt".
This can be resolved by recreating the user and placing them in a Security Profile again - at which point all of their records will return to normal.
If the user was deleted because they left the company please look for How to deal with a user leaving the company.
If you are unsure of which users have been deleted, or do not believe any users have been deleted, please raise a new case online and reference the title of this article.
Error - 'ExecuteDataLink: Error handling request - Object reference not set to an instance of an object.' when editing a WorkFlow Form record
Updated over a week ago