Hi Martin,
Thank you for the details.
That is very unlikely, I suspect the issue is somewhere in the post process code. I see you are using the new PDF images feature. I think that might be running into an error somewhere, as it is a fairly new code.
I tried to log-in to test, but wordfence does not allow me to visit the back-end URL. Can you please check that, and also add temporary FTP access? I would like to debug the image generator to see where it fails.