Hey Stephenh,
So sorry to hear about the frustrating experience and sorry for the delay in response to your query.
As mentioned that you have almost tried all the troubleshooting suggested and it still doesn't makes any difference. So would that be possible for you to share the file that is giving error on conversion?
"You can share the file with us via email. And you can send it to the email that is shared via private message. Please do mention your forum thread in the subject"
Please refer to the related discussion here- Re: Conversion Failure and also check with the different pdf file to see the whether it works or not.
Regards,
Akanchha