Maybe it's slipping too much to begin with because there isn't sufficient tension, or the grunt of the polini is too much and just slips the belt off the line? That would chew through them pretty quick.
It's possible that texturing the pulley surfaces could help the belt stop slipping and eating itself.
Maybe you could increase belt tension a bit by getting creative with the pulleys/variator.
On top of making it easier to start, running one ring would tame it a little and maybe it wouldn't be so hard on the belts.