Hey Karls,
From what I can find, this error has more to do with SSL configuration/setup than the PDF generation itself. This might be something related to the local SSL certificate, or the SSL certificate of any of the assets (images, stylesheets, javascript, etc.) being used in the HTML being converted by the plugin.
The first thing I would try is running the Test Server Requirements action from the plugins page to potentially narrow down if this is an issue on the server, or with something in the PDF you're trying to create. If all the tests pass, then I would start looking at the assets in the HTML you're converting to see if any come from other domains which may be having SSL issues.
I hope this helps point you in the right direction - let me know if you have any other questions!
Thanks,
Daniel
Technical Lead
interactivetools.com