Issue with the microcontroller UART1
It's not a good idea to leave RX pin floating. You need a pull-up resistor on that pin, either internal or external. Otherwise, a falling edge on that pin is considered as a start bit. I don't know anything about your firmware, but it's possible that it can be blocked if it receives random characters. Sometimes floating pins are affected by the logic states of their neighboring pins.

Interface ICs like MAX485 or MAX232 often have internal pull-up resistors on these pins. Also, in most cases it's a good idea to add a pull-up resistor on TX pin too.