This kind of error is caused by insufficient privileges of the user which granted access for A5 Fax app, i.e., the user cannot access the files which are used as Sent Fax record Attachment ID fields if they are created and owned by another user, and user’s profile doesn’t have permission Query All Files.

This can be fixed by creating a permission set which adds Query All Files permission, and assigns it to the user which granted access for the app.

You can find this permission in the permission set under App Permissions —> Content —> Query All Files, and assign it by clicking on Manage Assignments and selecting which users to assign to.

You can learn more about permission sets by following this link: https://help.salesforce.com/articleView?id=perm_sets_overview.htm&type=5