Are you by chance using the new Workbench IDE? Some strange stuff happens if you have both it and the standalone CLI installed, at least on my WIndows box. I saw exactly that same message today. (I think this issue has been fixed in the latest installers though.)
But – FWIW I never use the serial monitor in CLI; I use TeraTerm (Windows only) . Unlike CLI’s “listen only” capability, TT is of course bi-directional, so you can interact with a Particle device when it’s in Listening mode to do things like get the Device ID, set up wifi, etc.
One thing that I really like about it (and the CLI serial monitor can do this) is that it will maintain a serial port session with a device that is restarted.
