I tried to monitor Arduino COM3 with the software 'PORTMON.EXE' without success. Then I made a modification to my project in progress and Arduino reported the following error: "COM3 cannot be detected". It was necessary to restart the PC for Arduino to recognize COM3 again.
"COM3 cannot be detected"? Pretty sure that was not the exact phrasing of the error you got. Can you provide the full text of the error message?
Is there a reason you need to use portmon instead of some other more "normal" serial terminal?
PORTMON balks at a lot of X64 systems and can be a pita.
Was great in its day but has been surpassed by a lot of other utils.
It was also known to hold onto COM / LPT ports explaining why you had to restart.
If you have it set to AUTORUN it could still claim ports etc. and stop some software from working.
Bob.