I have no idea what 'Sandbox' is and the closest result to what I can assume you're talking about is 'Sandboxie' which can cause VAC errors.
Sandboxie blocks VAC, that's the problem.
Either try run Sandboxie and Steam both as admin, and then go to the CSGO files;
Local Disk / Program Files (x86) / Steam / steamapps / common / Counter-Strike Global Offensive
and run CS:GO from there as admin. Check if that works, if not;
Download a Virtual Machine, run Sandboxie from that. That should work, and once again, if not; that confirms Sandboxie blocks VAC.
Hope this was of some assistance.