Thanks, @trentmu. I found wkhtmltopdf working. The error stricly only occurs when the mail is picked up from the crontab job running
$ cd /home/frappe/frappe-bench && /home/frappe/frappe-bench/env/bin/bench execute frappe.email.queue.flush
This is the case as the automatic sending is broken after v11 upgrade (see also V11 can't suspend nor resume email queue). While the resume button is back on in v12, it still has no impact… If the exact same mail is sent by pushing “Send now”, it works…