Can anybody explain what it means when the port LED stays lit red when I plug a new serial module into the NC-50?

I'm trying to add a new device and that light stays on and my NC-50 won't operate properly. I go through "Find," etc. and it says the NC-50 was updated successfully, but it just wants to keep cycling through the Find procedure. It won't actually send commands to any of my devices. None of the other lights with attached devices stay lit like that, so I'm just wondering what the light indicators mean...

TIA!