Server Version#: 1.21.4.4079
Player Version#: 5.1.0
Since few days ago, subtitles started showing both normally and in black box. Even when I disable subtitles in plex, they still show up in black box. It is happening with every media file.
Sounds like the black-boxed subtitles might be burned into the video which means you cannot disable them. Which will of course make the soft subtitles redundant.
Can you confirm this by opening the file in some other player (e.g. VLC)?