HP Harlequin rips generating error processing PDF file

Hi all,

I'm a programmer for a company that prints books based on customer uploaded data that we use to generate PDF files. We process the PDF files for printing on HP Indigo presses, which use the Harlequin rips to rasterize the PDF file for the press. Occasionally a PDF file fails to get ripped, and the system generates an error file about why it failed. However, this file is not very precise about what the problem is, which makes it difficult for me to fix our upstream process that generates the PDF files. Here is a sample of an error file generated by the rip:

Starting Job On Friday, December 14, 2007 4:33:02 PM
Using Color Setup "HPIndigoPhotobook_sRGB"
FontEmulation enabled
Using device calibration from profile `HPIndigoPhotobook'
Ripping file %SRS-NAS02/prerip%/ripinput/SRSRIP9-1/Press5/IndigoPhotobook/book_fil e.pdf as PDF.
PDF version 1.5
Title: Not Available
Subject: Not Available
Author: Not Available
Keywords: Not Available
Creator: Not Available
Producer: PDFlib+PDI 6.0.2 (Python 2.3.4/Linux)
CreationDate: D:20071214162047-05'00'
ModDate: D:20071214163200-05'00'
Trapped: Not Available
%%[ Warning: Font Arial Bold emulated. ]%%
%%[ Error: stackunderflow; OffendingCommand: pop; File: %ip:Press5_IndigoPhotobook% ]%%
%%[ Error: invalidrestore; OffendingCommand: Do(PDF); File: %ip:Press5_IndigoPhotobook% ]%%
Total time: 0 seconds
%%[ Flushing: rest of job (to end of file) will be ignored ]%%
Job Not Completed: book_file.pdf

In general all of the errored jobs have lines in the error file about this OffendingCommand: Do(PDF); but I have no idea what this means. If anyone has any ideas, insight, suggestions, I'd very appreciative to know them.

Thanks in advance,
Doug Farrell