I have the latest update of LR CC running on an up-to-date iMac with 16GB of RAM.
I have a well established catalog with new and older videos installed. Most are MOV files. When I select one, in the Loup view, a preview image appears and the Play button. when I click Play I get a pause button and no change in the preview image. No change in the position indicator, nor the seconds played information.
When I try to change to another video, the preview image does not change, and stays with the original video.
Any suggestions? the photo handling looks fine.
First, verify that you are on CC 2015.3 -- "the latest" is ambiguous, since the Adobe updater often fools people into thinking they have the most recent version. Do Help > System Info to verify.
Second, add your vote and details about your issue to this topic in the official Adobe feedback forum: Lightoom 6.3: After update I can't open any video files. Three Adobe product engineers are actively engaged in that topic, and best for you and Adobe if you continue the discussion there.
Thanks for the information. Much appreciated. The issue in the link you provided is much more advanced than my question. I will pursue the issue there.
For others following, they seem to have a solution or at least a work around.