Thonny users found that after upgrading MicroPython on their ESP8266 from 1.13 to 1.14, the WebREPL connection didn't work anymore (https://github.com/thonny/thonny/issues/1613).

Thonny first tries to communicate with the device in raw-paste mode, but this fails at websocket level when the device actually supports this mode. It looks like WebREPL is sending the window size increment as a text frame and this can produce a UnicodeDecodeError. Example stacktrace when using ESP8266:

Traceback (most recent call last):
  File "C:\Program Files (x86)\Thonny\lib\site-packages\websockets\protocol.py", line 827, in transfer_data
    message = await self.read_message()
  File "C:\Program Files (x86)\Thonny\lib\site-packages\websockets\protocol.py", line 910, in read_message
    return frame.data.decode("utf-8") if text else frame.data
UnicodeDecodeError: 'utf-8' codec can't decode byte 0x80 in position 0: invalid start byte

I didn't check the actual content and type of the websocket frames, but I've seen 0x80 as part of the window-size increment bytes before.

For now, I'm going to use regular paste mode with WebREPL connection, but it would be nice, if the more efficient raw-paste mode worked as well.