Default Scanline. I believe right up until the latest version, it was the only renderer that was supported in RTT - with Arnold supposedly being supported now. I've never used Arnold but I tried switching to it for RTT and 3ds Max just spat out an error and crashed.
I was going to suggest switching to scanline as this is clearly some sort of horrible bug. I can't test it for myself atm but would be happy to once I get to a computer. Arnold will probably have a separate set of commands for RTT - it does in maya. If it's anything like in maya you'll probably have to knock up a little…