OctoPrint-AckAckAck. Install by placing octoprint_ackackack.py in ~/.octoprint/plugins or (OctoPrint 1.4.1+) by pasting "https://gist.githubusercontent.com/bigwheel/4ed84496d88faac7aeefeeb823fea300/raw/octoprint_ackackack.py" into Plugin Manager > Get More > from URL

octoprint_ackackack.py

# coding=utf-8
from __future__ import absolute_import

import octoprint.plugin
import logging
from collections import deque


class AckAckAckPlugin(octoprint.plugin.StartupPlugin):

    def __init__(self):
        super(AckAckAckPlugin, self).__init__()
        self._comm_tail = deque(maxlen=10)  # Buffer 10 lines

    def on_startup(self, host, port):
        from octoprint.logging.handlers import CleaningTimedRotatingFileHandler

        path = self._settings.get_plugin_logfile_path()
        logging_handler = CleaningTimedRotatingFileHandler(path,
                                                           when="D",
                                                           backupCount=10)

        formatter = logging.Formatter("%(asctime)s %(message)s")
        logging_handler.setFormatter(formatter)
        logging_handler.setLevel(logging.DEBUG)

        self._logger.addHandler(logging_handler)
        self._logger.setLevel(logging.DEBUG)
        self._logger.propagate = False

    def on_gcode_received(self, comm, line, *args, **kwargs):
        self._comm_tail.append("Recv: {}".format(line))

        if "o\\x00k" in line or "o\x00k" in line:
            self._logger.error("Garbled OK detected:")
            self._logger.error("-" * 80)

            for l in self._comm_tail:
                self._logger.error(l)

            self._logger.error("-" * 80)

            return "ok"
        elif line == 'k\n':
            self._logger.error("Garbled OK detected: k")
            self._logger.error("-" * 80)

            for l in self._comm_tail:
                self._logger.error(l)

            self._logger.error("-" * 80)

            return "ok"
        elif "\x00" in line:
            self._logger.error("Potential garble detected:")
            self._logger.error("-" * 80)

            for l in self._comm_tail:
                self._logger.error(l)

            self._logger.error("-" * 80)

        return line

    def on_gcode_sent(self, comm_instance, phase, cmd, cmd_type, gcode,
                      *args, **kwargs):
        self._comm_tail.append("Send: {}".format(cmd))


__plugin_name__ = "OctoPrint-AckAckAck"
__plugin_pythoncompat__ = ">=2.7,<4"
__plugin_implementation__ = AckAckAckPlugin()
__plugin_hooks__ = {
    "octoprint.comm.protocol.gcode.received": __plugin_implementation__.on_gcode_received,
    "octoprint.comm.protocol.gcode.sent": __plugin_implementation__.on_gcode_sent
}

https://gist.github.com/bigwheel/4ed84496d88faac7aeefeeb823fea300#file-octoprint_ackackack-py-L44
elif line == 'k\n':

This seems to be the issue for me as well.