It's great that you've made the switch from a wireless gateway to a serial gateway. Regarding the error LED flashing, there could be several reasons behind it. As you mentioned, when your Wemos Mini was connected to your PC, you were able to log via the USB port to identify any parsing errors.
Since the serial gateway is now directly connected to your Raspberry Pi, you'll need an alternative approach to log the communication. One option is to use a serial monitor software on your Raspberry Pi. This software allows you to view the data being sent and received through the serial connection, helping you identify any potential errors.
You can install and use popular serial monitor programs such as Minicom or PuTTY on your Raspberry Pi. These tools enable you to monitor the serial communication and capture any error messages or unusual behavior that might be causing the LED to flash.
Remember to configure the serial monitor software to match the baud rate and other settings of your serial gateway.