Its Unraid. But thanks, after I looked a little closer at my docker setup, I found the problem. Even if I don't quite understand what exactly was going on.
For anyone who might have the same problem and sees this in the future:
Additionally to the two paths in my last message, I had this path configured:
I think that was all in the default configuration of the docker template "bbergle-jellyfin". But if I already configured a path to the folder where "Movies" and "TVShows" are in, it doesn't make sense that I configured it again for the subfolders. After I removed the paths to the subfolders and only had "/data", the folders with lowercase didn't appear anymore.
For anyone who might have the same problem and sees this in the future:
Additionally to the two paths in my last message, I had this path configured:
I think that was all in the default configuration of the docker template "bbergle-jellyfin". But if I already configured a path to the folder where "Movies" and "TVShows" are in, it doesn't make sense that I configured it again for the subfolders. After I removed the paths to the subfolders and only had "/data", the folders with lowercase didn't appear anymore.