SOLVED: Playback Error MKV

[Rubber9146]

What's the error specficically?  The whole 'server isn't sending compatible format' one?  Do you have transcoding enabled (even if you don't want to long-term, but it could be either audio or video codec incompatibilities and could help with troubleshooting)?  If there's any type of transcoding going on there should be FFmpeg.Transcode-XYZ.log files generated, or are there none of these?  What client are you using (firefox/ jelly media player/ etc)? What are the codecs for the file(s) you're attempting to play?  (you can look at this through either ffprobe or by navigating to the file and selecting 'media info' from the kebab icon).  I know that .mkv containers require remuxing depending on your client so a different container may fit your needs better if you want no muxing to happen (though I don't know your scenario so take that may not be a relevant factoid for your use case).

the error message is in the description of this thread, but like the one you asked in the 2nd question.
There is no FFmpeg log (I already tried to disable transcoding by disabling the rights for transcoding for the specific user).

I figured something out: The video isn't playing on jellyfin App for iOS but on Swiftfin. The video isn't playing on Brave but on Microsoft Edge. With my good old plex installation, it is running on all browsers and on the native app from plex.

My idea was (or at least hope) that I can turn off transcoding with these checkboxes turned off. If I turn them on again, I can play anything on the native app and brave.
So question will be, how can I stop it from transcoding?