Ever since a failed upgrade from 1.8.1 to 1.9.1 (rolled back now) coupon codes don't work for customers that are logged in. Works fine when not logged in. I just purchased an extension that lets me set customer error codes for coupons and when logged in the error is for the 'catch all' error - ie one that has no known reason.
I am thinking that maybe the file that controls coupon application is corrupt or somehow not successfully rolled back
Does anyone know what file applies coupons in the cart and checkout so I can re-install from a fresh down of our version?