error while slicing (no replies)

Hi guys. I keep having this error appear while slicing.

I am using the reprappro host software for multi-color/multi-material slicing running on ubuntu 13.04. I prefer using this software as it also prints out the supports and the nozzle cleaning bay.

This error occurs both when I am doing multi-color printing and when it is just single color printing. IT is getting in the way of my work! argh!