UPDATE: July 17, 2018
A security enhancement that reduces an encryption key's time to live (TTL) was determined to be the root cause of the problem. A patch release has been completed that minimizes the potential impact down to a two-minute window every eight hours. If you experience delays in previewing a video that are caused by this issue, please wait two minutes and try again. The preview function should work correctly. This issue is under ongoing research for a more complete resolution.
July 16, 2018
Description: Some customers report being unable to use the preview feature while editing a video.
Scope: All videos being edited that have been open past the required rotation of the video's encryption key.
Work-around: Wait until the encryption key has been rotated (a maximum of two minutes after the patch referenced at the top of this article)