I use the softer Polar strap and I get the same rash. I haven't used my hard strap in a long time so I can't say if it also causes this issue, though it didn't in the past. However, I'll admit I'm lazy and don't wash it after each use. Once I wash it, I'm generally rash free. Sometimes, I'll still notice some red bumps, but they don't really itch and go away quickly.

I've got the Polar bra and it doesn't work for me. At least, not for the purpose intended...as a boob holder, it's great, but the band intended to hold the strap doesn't hold it tight enough against me to pick up my heart rate.

Ok, so I have no real solution for you. Just wanted you to know it happens to me, too.