I've had a similar issue but in different circumstances, so this may not be an answer but worth a try. I found the invoice layout was using a template that was no longer accessible, so there were very long delays and memory errors producing an invoice.  The solution was to open Report Designer for the invoice layout, choose View and Properties and remove the template.

I've had a similar issue but in different circumstances, so this may not be an answer but worth a try. I found the invoice layout was using a template that was no longer accessible, so there were very long delays and memory errors producing an invoice.  The solution was to open Report Designer for the invoice layout, choose View and Properties and remove the template.