Hi,
I just experienced this same issue. I use the Proxmark a lot and found a few things might be wrong.
- If you were making edits to the code like I was you need to redownload the proxmark flash files from the website.
- if you have a blueshark attached, remove it from you proxmark.
- go to your device manager settings once connected and force it to stay on COM 8 in the advanced settings.
- also try a new cable. I somehow managed to break my cable in the process. But once I did all of these steps I was able to reflash the proxmark with the new downloaded scripts and it works again.
I hope this helps.
ZRG