Movies showing screenshots instead of posters

@Papageno_1961 said:
No to IPv6.

Good.

Antivirus: nothing but Windows Defender that comes standard with Windows 10. No change there.

Tell it to stay out of your Plex data folder anyway.

You’d think the program could just grab a couple of them from the videos themselves. No matter whether I classify them as movies, other videos*, etc. do I get any images. *in the latter case, would I use a Personal Media agent?
My next strategy is to grab all the problem videos and move them to their own folder, create a separate library for them, do the “Plex dance”, clear caches and see if any of that helps, I guess.

Yes, try a ‘Other Videos’ library. This one relies on automatic thumbnails-only.

EDIT: BTW, are the thumbnail images that appear when you hover the mouse cursor over the “progress bar” (for want of a better term) in some subfolder of the Media subfolder?

They are stored in the Plex data folder
Filename extension .bif, very hard to find. But they won’t help in your case, I’m afraid.
https://support.plex.tv/articles/202197528-video-preview-thumbnails/