What Phil said is valid, but because serial is reflected in debugger when you use a loopback on the DM-RMC this suggests there may be another problem.
Check that you don't have any ground voltage difference between the display and the DM-RMC. This could cause a latching in the coms chip. Remove the HDMI cable and the serial and measure, it could be AC or DC or both. Use a scope to evaluate if there is any hint of a potential.
Check the flow control settings, are they correct for the display.
If retrieving the code corrects the issue, are you sure that there is nothing in the code that is getting into a state that is affecting the RS232 communication with the display? I'm not familiar with the Promethean API; does it require an AK of some sort, could this be getting out of sync etc.
Lindsay
