From what I understand - and i could be totally wrong, so take this with a grain of sand: The rip-to-image service is used to convert graphic assets into a format which is more efficient for the document composition engine to insert into the output.
So, you may have a bad graphic in your assets folder that is failing this conversion.
What type of assets are you using, and what output format are you creating? Let me explain why I ask:
Early versions (previous to v5.2 or 6.2 - I can't remember which off hand) XMPie recommended that assets be EPS format. Because the way it generated output was based on PostScript and EPS is quick and efficient to insert into PS.
After that, version, there was a change in the way XMPie creates PDF and PDF/VT-1 output to be native rather than using postscript as an intermediate format. Therefore, EPS assets had to be converted in order to be inserted into the PDF output. (VIPP/PS/PPML/VPS output formats do not require this conversion.) So, like I said, it depends on the output format and the asset types as to whether this rip2image service is used or not.