This can occur when Attache freezes or drops out and user is disconnected. When the user logs back in and tries to create an invoice the "Could not open temp file" error occurs. Other users are also getting the error when trying to create invoices.
Try the following:
Log out of Attaché.
To clear the Attache temporary files, see: Attaché: How do I clear Attaché Application cache application files
Restart your PC.
Log in to Attaché to confirm the error is resolved.
If the error is still occurring after perfoming the steps above, please raise a new case online and reference the title of this article so the support team can assist.