I tried those potential fixes yesterday. Neither worked.

After deleting the attachments, I still got the error code and the attachments (which are auto-generated) came back.

In the Temp folder referenced in the error message, I found 20.pdf and 21.pdf, both of which related to the return in question.  I tried just making a copy of one of those and renaming it 19.pdf, but got the same message that the system couldn't find the file.

I ran the PDF repair utility in the Troubleshooting screen - still didn't work.

I got through to Lacerte Support today.  We went through a bunch of tests but what finally worked was going to 

Settings -> Options 

Under the "Program Options" section, we changed the "Auto Generate and Attach PDFs to EFiles" option to "No".