I think that's the real source of the problem here.
The addition of the K-9 lock and the "fatter" points on the K-9 lock
are using up alot of the length of the screw.
If you push real hard, no problems as the screw will drive the points
into the nylon webbing of the footstrap and then compress the entire
assembly (K-9 lock/nylon webbing of the footstrap/Oval toothed washer)
and give you plenty of thread engagement.
If you fail to press down on the screw enough to get it to fully engage and
compress the assembly, the screw will "ream" the upper part of the plastic
So, my suggestion to "pre compress" the assembly and get those stubby points
up into the nylon webbing of the footstrap.
As I've stated, this won't be a problem in 2009 as the screw length is being increased
from 28 mm to 32 mm.
I've had no problem with the 2008 boards that I have, once I figured out that I need to "push real hard" on the screwdriver to get the threads to engage correctly, or use the "pre-compression" method with a fin screw, a 6 mm x 1.0 nut and a couple of washers.
Hope this helps,