Grey previews are a symptom of either a bad monitor profile, incompatible video driver, or a corrupted preview cache.
What display profile do you have assigned to your monitor? Usually ones that come from the monitor manufacturer are corrupted as far as LR is concerned.
As Ssprengel said, the grey rectangles are often caused by a display profile that's incompatible with LR. Windows 10 in particular seems to have a nasty habit of installing such profiles, but sometimes it happens on Mac too. See this article for how to troubleshoot that easily: http://www.lightroomqueen.com/articles-page/how-do-i-change-my-monitor-profile-to-check-wh ether-its-corrupted/