I'm sorry for making the most common mistake by not using the search function of the forum before writing my question.
In doing so finally I found several ways to try - so I tried and it worked, so no help is needed anymore here.
If anyone has the same problem like I had:
A. Try Removing and Adding back monitored folders
1. Open windows media player
2. Go to Library
3. Remove the folders that Windows Media is watching for content
4. Quit WMP11
5. Re-open WMP11, and add your music folders back
B. Try Moving your library
This should reset the permissions for the content.
1. Create a new folder
2. Move your content out of its current folder
3. Then, move your content back into the correct folder
C. Try removing music from library only. Then importing again.
1. Open windows media player
2. Go to Library
3. Remove all tracks from the library, only the library.
4. Quit WMP11
5. Re-open WMP11, and add your music folders back
(quoted from
http://forums.rokulabs.com/viewtopic.ph ... ht=napster)
Cheers,
AquaLume