The only time I have seen spokes break at the nipple was when I built a rear wheel for a friend with a Power Tap hub. The hub required more dish and the left side spokes did not have enough tension when I had about 105kg on the DS.
I ended up tensioning the drive side up to 130kg so as to have enough tension on the NDS. No more problems.
My friend is about 225 pounds and a gym rat, so he is very strong.
The spokes are double butted 14 gauge.
I am wondering how much tension you have on the spokes?