That might be a Windows user privileges matter.
Perhaps things were installed as administrator, or you are logged in as a user with limited rights, etc.
You should be able to find a solution for that on German-language Windows fora?
PS
Why not update to 3.5? You are using a rather old version.
You didn't say if you are using 'installed' or 'portable', but if it's an option for you, use a portable version, and install it somewhere outside of Windows' system folders. (e.g. on D: , or C:\MusicBee)
The same for the music files themselves.