Ah, well, this one means just that: the pdf file was empty, and therefore wasn't produced since it has 0 pages.
A 0 pages pdf file could have various causes, the most common one being some page conditions that would all be evaluated to False for some reason, thus producing an entirely empty job. This could happen if data has moved, and information that was supposed to be at one place is now on a different line. Of course, that's one example, there could be other causes.
You say that it started happening ever since you switched to Optimized PostScript. That's a bit odd, though perhaps the data isn't emulated correctly in Optimized PostScript... Do a test: take the latest data file that has failed, open your form in PlanetPress Design and load that data file. Then preview in both Printer Centric and Optimized PostScript, and see if you get an error, or a different result with one mode or another.
Regards,
Raphaël Lalonde-Lefebvre