Problems with previews: missing, grey, green or black screen instead of video preview, slow previews

Possible solution 1

1. Update your operating system & Adobe products
2. Windows Only: Make sure to update your graphic card drivers. Use only drivers downloaded from your graphic card vendor (Nvidia, AMD), do not use 3rd party driver installers (like Driver Booster or similar).
3. Restart computer

In our experience this issue is usually caused by bad drivers on Windows. On Macs usually the solution 2 below helps. 

Possible solution 2

Close all other extension panels. We had some reports that some extensions cause our previews to misbehave. This happens when other extension refreshes UI often or does some calculations while idle.