Before you redo the rest by hand, check whether the broken ones share a format. Black frames that render fine on the phones but not in a desktop browser usually mean the original file is intact and it was the server-side preview that failed to generate, with the phones quietly decoding the original themselves. HEIC and HEVC and 10-bit HDR are the usual suspects, since those lean on libheif and ffmpeg on the server and older builds choke on them.

If that’s the cause, re-uploading only “works” because it reruns preview generation, so you’re really just triggering a fresh encode. Grep the museum logs around one of the bad files for a decode error, and see whether a rescan or clearing the thumbnail cache regenerates them in place. That would save you doing the whole library one file at a time.