Videos at forum displaying as black boxes is, unfortunately, new & “normal” behavior of forum software core. We reported that to forum developers in attempt to make this change optional. Their justification to remove video previews globally across thousands of forums around the web is to prevent browsers in some cases to load the whole video in attempt to display the video preview image. I haven’t run into such behavior of my browsers, that’s why I find this change very strange, and hope they will be able to either roll back these changes or make these optional.
This does not change any other functionality with videos as these continue to be playable by all the devices. So, in case something doesn’t load for you at forum while there are no other reports by other users, this may indicate problems on your side only.