I have also turned off the McAfee and run GSX with no success.
Disabling an antivirus is not very useful, that's why we never suggest to Disable, but configure it to add the Addon Maneger folder to the Exclusions.
Antivirus are never really disabled. Unless you uninstall them completely, their low-level service is always running and we confirmed multiple times that, if a bugged antivirus is fixated to flag something as a threat, it will block it even when it looks "disabled".
McAfee is one of the worse offenders when false positives are concerned, and they intentionally crippled the free version without the ability to add folder Exclusions only to give you an incentive to upgrade to the commercial version.
If you want a free antivirus, just use Windows Defender, works perfectly fine.