According to Adobe, this error point to an element in the lay-out that poses a problem for the RIP (isn’t that obvious!?!).

This could be an imported graphic but the error is most frequently caused by a damaged font. Try replacing your fonts and open all images and resave them again in the originating application.

Nostringval errors can also point to problems with a hard drive of the RIP or problems with the available disk space. Check to make sure enough space is available for the RIP.