The latest Garmin "premium" strap has been known to cause severe chafing issues for some of us. My own chest ended up bloody after a few long, sweaty runs and I still have scars left from it. Luckily I found a quick, inexpensive and durable fix for it.
These pictures show how I fixed it using a piece of thin mouse pad. The piece that I cut out is 10 x 5 cm (4 by 2 inches) - but you may want to experiment.