Because you're already connected to the Slingbox, and the error occurs only when you power on your cable box, it sounds like that error might be coming from your cable box itself. The first thing I would try is unplugging your cable box, and plugging it back in.
Would you be able to include a screenshot of this error message? It might help us tell for sure where the error is coming from. I would also be interested in knowing if this error happens at home also.
Hope this is helpful!