i have used the moxa single ports, 5100 series for years on many many shows as serial device servers for RS-232, 422, and 485. I have never had them be inconsistent. You said you changed a port number on a projector, you are connecting to the projector with a 9 pin serial cable correct ? or were you talking about the port on the moxa ? to use WO with the moxa i would use TCP, set the moxa as a tcp multi client server and the serial settings required for your projector. often times the serial port of the projector is less configurable than these device servers. you may need to try different flow control settings for example if they are not specified in the projector manual.
i have used these with fixed cameras, robo cameras, switchers, matrices, projectors, monitors, relays, and more. I guess you could have a bad one but if it is a dual serial port and working on one projector there may be a different problem. what happens when you cross plug them ?
gpeeler