Ok, i found the explanation. The difference between the 2 situations was the use of my vodafone usb modem, which, when the software is activated, creates to COM ports, COm 3 and COM 5.
No real idea. Intermittent problems can sometimes be associated with software, boot sequence, drivers and hardware. I'm guessing some USB item did not register before starting FMS on the time you had an error. But only guessing... -- ggunners