In my case I think it has to do with the encoding of the filename and the locale+encoding on the operating system. My image files could contain the swedish letters ??? which causes a problem if the system doesn’t have that locale.
I got the error message “The field couldn’t be renamed from old/filepath/ to new/filepath” so I dug into the plugin just to learn more about that message. In the file media-file-renamer.php on line 1043 there’s this check:
if ( ( !file_exists( $old_filepath ) || !rename( $old_filepath, $new_filepath ) ) && !$force_rename ) {
I printed out old_filepath and instead of the ??? letters i saw utf-8 literals like this: “path/imagexc3\xb6.jpg” when it should be “path/image?.jpg”.
This was on a debian machine. I then tried it on my mac and it worked and the output of the filename was correct.
Bottom line is I think the problem has to do with the encoding. Encoding for me has always been a headache ??