Error retrieving InvoicePDF

Hello Experts,

While checking SLG1 log I am getting few errors like

Error retrieving InvoicePDF: InvoiceDoc 03******1 URL for object 084******380 (CA_DOC)

Error retrieving Invoice for user C*****23, BP 0*****1 (return code 0, response 0)

Assume these stars are any alphanumeric value.

Could you please tell  me what is the root cause of error and any solution.

Thanks & Regards

Amit