FYI, I modified a copy of the Plex Media Server binary to call the Plex Transcoder (which is really just a fancy FFmpeg) with "-skip_frame:v none " instead of “-skip_frame:v nokey”, and it worked.
cp -a "Plex Media Server" "Plex Media Server.orig"
sed "s/nokey/none /" "Plex Media Server.orig" > "Plex Media Server"
Obviously it’s a lot slower, but if the only files in your library are these ones, it gets the job done until a real fix is available.
Edit: scroll down to a lower post for a much better solution.
Video Preview Thumbnail Generation Fails for Some Items - #48 by Volts
