SOLVED: AV1 + subtitles doesn't work with hardware acceleration enabled

[TheDreadPirate]

What are your server specs? That message probably means you check "AV1" in the transcoding settings, but your server GPU doesn't support AV1 decoding. The fact that your client played AV1 only means that the client supports AV1. The check boxes in the transcode settings are for telling Jellyfin what codecs your SERVER GPU supports decoding.