that is the same area the Gilles Berthoud Aspin bothered me and I decided it was the way the saddle sloped on the sides. The B17 has a more gradual slope the the GB.
I moved to the Rivet Pearl - same width but with a cut out and a gentler slope on the sides and have zero issues.
Did you check your saddle height to make sure you aren't rocking side to side, even a bit of rocking with hot and sweaty is going to cause a rub. Have some who has good knowledge ride behind you and watch your hips.