So I’ve tried building it again, and now it works (although it doesn’t play one of the videos, but that’s probably a bug in the app itself, will check later).
Possible fixes:
- I updated Jellyfin today from 10.10.5 to 10.10.6
- Apparently, even with buildRustPackage, I don’t need cargoHash. I had it empty and it never gave me the proper hash. I removed the attribute, and it didn’t complain at all lol
- I had a few nix files, trying different approaches at the same time. Removed all of them and renamed the file I built now. Could the result be linked/cached to a different build result before, even though I rebuilt it? That’s why it didn’t work no matter what I did?
- My nixpkgs input is 24.11 which probably changes every hour, so maybe some dependency got updated in the meantime? Not ideal, should’ve used a commit
Here is the code if you want to have a look code.
The patch is because of this. I should probably comment the code more, but I was just experimenting, so… 
Either way, it works now, thanks for the help! I’ll probably ask for more haha