How are you advancing the canvas to make the cut? Down arrow? Manually pulling?
Sometimes my 9900 gets confused about how much it can back up, then I manually grab the media roll caps and twist them to pull the media back through. There's quite a bit of friction to turning, but I've never damaged the media doing this. Not sure if the 4900 is spindleless, but I'm sure there's still a way to force it.
If you use automatic media length tracking (I don't) this will surely screw with it.