ok some suspect recent changes in History for frappe/email/queue.py - frappe/frappe · GitHub
Most notably this fix communication: fix attachments in email queue and generated reports by Thunderbottom · Pull Request #6586 · frappe/frappe · GitHub
Otherwise no error it seems that any pdf generate process is part of the problem, but confirm that? The general case converts an HTML document format I believe.
You can run wkhtmltopdf from a console for eg
Steps to install wkhtmltopdf with patched Qt (version 0.12.3) (for pdf generation)
As well wkhtmltopdf must have http access to the html file, and so the FQDN domain url must be specified in /etc/hosts and sites/exampledomain.com/site_config.json