I also use Norton (I'm forced to as I use a shared computer and the owner of it insists that Norton is indeed the best antivirus and we should always stick to it) and ran into this issue repeatedly. The only way I know to fix it is to go to Norton's settings, click antivirus, go to the scans and risks tab, click the configure button next to "Items to Exclude from Auto-Protect, SONAR, and Download Intelligence Detection", and add your GameData folder to the exclusions list. This is apparently a common bug with Norton and the only way to fix it is to restart your computer.