Full Error Message:
Error occurred on updating delivery status of Sent Fax record in Salesforce invalid cross reference id
Organization ID: 00D1N000078fDUA0
Organization Fax Number: 8663070000
Fax SFID: a1l3m000003HV85AAG

Reason for error:
This error can happen if the Sent Fax record ID cannot be found or was deleted. You may have deleted a sent fax record for example after sending (before status on fax changed to “delivered”) so the fax app cannot update the status on record since it cannot be found. Another reason you could see this error is if the user does not have access to the sent fax record due to permissions and the status cannot update.

Solution: 
Search for the sent fax ID in your org (in this example you would search for “a1l3m000003HV85AAG”) to see if exists or was deleted. If was deleted, you could retrieve it from the recycling bin so the fax status can update. If unable to recover the fax record, the app will continue to try and update the fax record for a period of 24 hours so you may receive more than one error message referring to same attachment ID. The error notification should stop after 24 hours.