Found this thread searching for the exact same thing. And this seems to be the only relevant result when googling this issue.
The message is "unknown protocol: c", and it appears in the output part of the IDE (black frame under the code editor), with red text.
It shows after sketches are loaded in the IDE. Not after compiling or uploading the sketch.
Everything seems to be working, but it gives me a bad felling not knowing what this means...
I have this trouble too.
Britesc, If you had fix this now please tell me how.
Thanks
====PROBLEM SOLVED======
The problem was in additional board manager URL.
VS code arduino extension had added the sketch book location to additional board manager URL because the arduino.additionalUrls property in settings.json file of VS code had set to my sketchbook location. But actually I didn't set it to that location with my concern. However I empty the arduino.additionalUrls property in settings.json file of VS code and removed that folder path from arduino IDE preferences. Then problem solved.