I now stumbled over this problem a lot of times, it doesn't happen every single time but pretty often, I don't really know what triggers this bug.
Every time you try to zoom in with your scope for a short amount of time it is black until you can see through the scope.
The time the scope is black can mean death when you peek around a corner and the enemy is able to see you before you see him or you just need to aim fast.
Scope Problem Insurgency
(the Youtube video is not listen on Youtube and only accessible via this link)