You probably overtightened it setting up the fracture. It's likely your shim was off size a bit causing you to feel you need more hold than it was designed for. Check dimensions carefully when you next install it (the shim has to be very close to 1/16") and if you feel you need more hold, use a tacking agent like the stuff 3m makes for light hold Post-it notes (comes in a stick), or carbon assembly paste.

The nut system is designed with exactly your problem in mind, just set a small punch against it through the clamp and tap it out the back. Odds you can remove what's left of the screw once it's out. You have a choice of buying an original screw, or using one made for water bottle bosses, or any stainless steel 5x.8 metric screw available at the hardware store.