Playback issue but transcoding works

[Peronia]

Hi, I'm using Jellyfin 10.8.13 via the repository on Debian 12.

When I try to play a video (no matter which) I see the background image with a loading circle (and after some time the screen went black, only on the Cromecast) but never a video. I see that the transcoding is working, files are created.
I don't know why Jellyfin isn't working anymore. I don't know when I watched the last video, but it works.

What I've recently changed:

• Added new media into the libary -> I don't believe that this might the reason
  
• Switched the transcoding path -> I reverted this change, now using the default path again
  
• Uninstalled an obsolete email plugin -> I don't believe that this might the reason
  
• Installed the plugin "Open Subtitles" -> I disabled it
  
• Installed the pugin "Reports" -> I don't believe that this might the reason
  

Since I can see the server creates transcoding files, I assume I have no problems with permissions.
   

In the logs I can't see anything abnormal:
Jellyfin log
Transcoding log

I've tested the playback with my Android handy, 2 browser (Firefox and Chrome) and a Cromecast.

Thanks for helping me.