Running MC25.0.114 on my rpi (buster). MC system info shows I have 50 thumbnails missing. However when I use options build thumbnails the action window says complete and doesn’t build any additional thumbnails. I noticed that on the first run that I got an error message that some folders/files were locked and MC couldn’t build some thumbnails. Don’t know why these files are locked, as I have not locked any files/folders. I suspect this is related to my upgrade to buster from stretch.
As an aside on my rpi that is still running stretch I have no issues. (As an aside I have a somewhat similar issue with cover art, in that on my buster version some cover art is missing compared to my stretch rpi, yet both pull from the same NAS).