So XBMC failed. I would have to install X-Server and several other stuff and it is cleary not suited for a standalone upnp server. Maybe sometime in the future I will connect the box to my TV and use XBMC as my frontend. But unti then, I will go back to MediaTomb.
No UPNP Server out there is perfect but MediaTomb is fast, reliable and easy to use (and I already have a config file from my last try).
So I installed MediaTomb and it works like a charm (after sometime it took for updating the library).
In addtion for anyone who doesn't have/want a UPNP Client I installed a PHP Directory Listing Script to just view the contents of thos dirs. (From here). This was easily set up. I put it directly in my media root /mnt/media and then excluded all directories/files that I didn't want to show up (downloads, some linux files and folders)...
And that's pretty much it. The media server is running and I think MediaTomb will be my final solution...