Summary
Invoice (DOC ID xxx) has been published to your accounting software but attachment could not be uploaded; please manually attach document in accounting software
If you are seeing this error after publishing an invoice, it would be due to limitations within your accounts package relating to the file size limit that can be attached externally. For instance, Sage Accounting will only allow you to attach files that are no larger than 2.5MB.
Resolution
To clear this error, you would need to save the invoice image to your PC, and manually attach it to the transaction in your accounts package.