I haven't seen this one posted in the forums, my apologies if it's already known. Since the last update, the thumbnails Plex shows for videos when fast-forwarding or rewinding have all just been black.
The thumbnails show in the Web interface and on my Roku, so they are being generated correctly. It also doesn't show the black box when doing this on a file that hasn't had the thumbnails generated yet, so it knows if the thumbnails exist or not.
Let me know if there's anything I can do to help (pull a log somehow, post a screen shot/video, etc)
Yes, this is an issue I’m having as well. It was working prior to the last update. The same title media index files will show fine on my roku streaming stick so this looks to be an xbox one regression bug.