Simply put, there are some tighter-than-need'be P4P V2.0 blades out there. Wrap a piece of innertube around the motor for grip and be sure to press and turn in the correct direction. It will come off with patience and the innertube piece.
I just sent my V 2.0 back to DJI for a fix for the exact same crack issue...on the two back arms, just forward of the motor. This is a fairly new Phantom and I was surprised as I treat it very well. I hope they fix it under warranty instead of using my one Refresh chip.