What are some common permission issues that can cause errors when sending or receiving faxes?

Make sure users have access in their profile or permission sets to Opero Fax app, all related apex, Pages, triggers, Sent Fax object, Received Fax object, and all fields on those objects. If you created any custom lookup fields on the Sent Fax/Received Fax object, make sure users have access to the related object.