Switch litter immediately. This is why corn cob litter usually is not recommended for rabbits. There are several products made out of recycled paper, although I personally prefer kiln-dried wood pellets. The kiln-drying removes the harmful natural chemicals that make pine and cedar shavings unhealthy.
It is possible for a rabbit to get an intestinal blockage from eating indigestible stuff like corn cob litter, same as with eating carpet, etc.