I’m having the same issue at my server as well, I’m using Version 1.7.2 Community Edition. I’m using https://purdox.com for my WordPress setup (I’m the author), it’s free and open source, a system to have your WordPress server on Amazon AWS in 5 minutes.
Somewhat the QR Code generation PHP has a problem, on the PDF instead of the image I see Image not found
I appreciate your help on this issue.
Kind regards,
Tony