Hi Robb,
Have you tried swapping the + and - like RJ said? That's a pretty common issue with RS-485.
If so, then just out of curiosity, I'd shut everything down and then check the resistance between the data+ and data- signals coming from the hardware board. Since it's working for you without the terminating resistor on the MR16 side, then there's a good chance that it's either not properly terminated on the transmitting end, or there's a cable issue.
If it's properly terminated, you should read around 120 ohms between the signal wires. (If you can manage to check it with the cables connected and both ends terminated, it should read around 60 ohms since the resistors are in parallel).
One other question just out of curiosity... You wouldn't happen to be using a really short DMX cable, would you? Generally speaking, for RS-485 they recommend at least a 3m cable to minimize reflections. I've used shorter cables without issue, but it's something to throw out there since its working better for you without the resistor.
Thanks for sharing with us!
-Murl