When the database is referencing one field that does not exist in the location it is expecting it to be, we may receiving the following error: System.NullReferenceException: Object reference not set to an instance of an object. If the error is followed up with any additional details we can look in those details for hints of what is being referenced. 

These additional details reference an export definition. The email process is referencing the export and does not see the object it is looking for. So in this case a user built an email with a merge field that no longer exists in the export definition used to create the email. Adding the field back to the export definition or re-creating the email resolves this issue. 

If this does not resolve your issue Chat with Support and reference this article.