I'm sorry I wasn't able to help you resolve the issue. I am using Windows 10 and have never encountered this problem with my MKR WAN 1300. I do occasionally manage to get my computer in the state where a COM port seems to be stuck open (perhaps because I closed the IDE while a failed upload process was running and it continued perpetually in the background?). I have always been able to resolve this by simply restarting my computer. It doesn't happen often, and it's usually when I'm doing wacky things like intentionally trying to get errors for testing purposes.