This happens because sometimes LinkedIn servers don’t load the thumbnail images in time, and display nothing (black) instead especially with the old videos.
Please note that just because you see this blank thumbnail in your LinkedIn inbox, doesn’t mean that others will see this. Oftentimes, the thumbnail image does indeed load and the end user sees it. You can test the link using LinkedIn Post Inspector
You can try the following to troubleshoot:
To try to re-process the thumbnail image in your inbox, try reloading the page. To reload the page you can click the reload icon on your browser or use a shortcut (Mac: CMD R, PC: F5)
Download the Dubb video, and reupload it. Make sure to apply the presets of the older video to the new one.
