Ok, the xml file itself seems fine.
However after closer inspection of the log, "System.UnauthorizedAccessException: Der Zugriff auf den Pfad "C:\Program Files (x86)\Team MediaPortal\MediaPortal\SubtitleDownloaders\OpenSubtitlesConfiguration.xml" wurde verweigert" stands out.
Are you sure the user running mediaportal has the correct access to that file?
Can you also please check if there's a copy of that file in the virtualstore somewhere? That happens (for backwards compatibility) when you write to a file in the programfiles as a non-admin user.
I'm running Windows 10, writing and creating a file in the Program Files folder is only allowed for administrators and MediaPortal is running from a normal user account without administrator privileges. So if the plugin needs write access to this file it will lead to an error, obviously.
i get that. at least in win7 it creates a file in the virtualstore for this scenario. don't know if win10 also has this compatibility-shm.
but as I understood from your logs, subtitledownloader only reads from that file. Can you check if a normal user is able to read that xml?