A clip with no picture is never a mystery in Vidmoat: the preview draws a labelled slate telling you which of three things it is.
"⚠ Media missing" with "Its upload never finished"
The file never made it to our servers. The clip was placed on the timeline instantly while the upload ran in the background, and the upload failed or the tab was closed before it landed.
The fix is to import the file again. Everything else about the clip is exactly as you left it: the timing, the transforms, the effects and the keyframes are all preserved and the new file drops back into the same slot.
Check the Media Library first. If the file actually did upload at some point, the panel offers a recovery banner reading earlier uploads available with an Add button per file, which is faster than finding it on disk.
"⚠ Media missing" with "no longer available"
The clip points at a file that has been deleted, most likely from the Media page. Re-import it, or delete the clip.
"⚠ Desktop-only file"
The clip was added from local disk in the desktop app and exists only on that machine. Open the project in the desktop app, or upload that file so it lives in the cloud. There is a banner at the top of the editor saying the same thing, and a warning badge on the clip.
"Replace this clip" on a checkerboard
This is not an error. It is a template placeholder. Select the clip and use Replace with your media: your file inherits the placeholder's look, position and animation.
Do not export in this state
An export would come out black for those clips, so the export refuses and names them. That is intentional. Fix or remove the clip and export again.
Timeline badges to look for
⬆ UPLOADING means it is still going and will resolve on its own. ⚠ RETRY UPLOAD in red is clickable and retries. REPLACE is the placeholder case above.
A black gap rather than a black clip
If the picture is black between clips rather than on one, that is a hole in the timeline. The pre-export check flags it, telling you how much black there is and where.