I've got a DT Swiss XR4.1d rim and I've noticed after breaking a few spokes that the nipples sit square in the rim so that the larger diameter of a Rohloff hub causes the spokes to bend as they enter the nipple.
I have just experienced the very same problem but with Alex DM18 rims. I have just gone through 3 spokes in the last 100kms and am becoming adept at quick spoke changes on the side of the road!!!
Yes, the larger diameter of the Rohloff hub, along with the smaller 559 mm 26" wheel diameter results in a very acute angle when the spoke exits the nipple.
Some rims have the spoke holes drilled in such a way that they are almost in-line rather than right/left/right/left. This also results in more stress than others.
I suspect too that the spokes breaking are the ones that exit the hub from the outside of the flange rather than those exiting from the inside surface - is that right?
One other option that hasn't been mentioned is buying shorther spokes and relacing the wheel using a 1 x pattern rather than the usual 2 x pattern. I suspect that it MAY be possible to have all the spokes exiting the flange from the inside face using 1 x.
Has anyone done this?
Finally, a useful innovation I have found, is replacing rim tape with little plastic spoke hole plugs. As well as rsulting in marginally less rotating mass at the wheel rim it means spokes can be replaced speedily by the side of the road!
Cheers from the arse-end of the earth...
Tony