Yep, done killed mine too. I actually ended up just pulling the rear cover over the screw. That side of attachment is now useless. I got one good screw and then I took athletic tape and went around the unit. Works just fine on the chest strap. Garmin CS was NO help on this one and it was annoying. I wanted to buy just the rear cover and a couple of screws and they said in a polite way, screw you! They don't sell the parts for it, told me I had to buy a whole new unit.
I am hoping the wife gets me a second unit for Christmas and then I will just use the "defective" unit for indoor training. I would forgo the chest strap all together but I like having that data on hard rides and uphill training.
If you do find a solution I would love to have it too. Good luck!!