I am also using Library Server on a remote storage computer to connect to several client computers with all using MC17.
It doesn't matter which computer is used to scrobble, they all show at Last.fm as soon as the track starts, (although unless at least half of the track is played then the scrobble disappears later).
But I only get one record of the played track, and MC17 correctly reports the relevent tracks played by two separate last.fm users sharing the same Library Server, but with different user names in the Manage Last.fm account settings on separate clients.
As suggested by AoXoMoXoA I have checked and both the Library Server instance of MC17 and each of the client instances. All are checked to submit playback information (scrobbling) so I do not think that is the problem.
Perhaps with your upnp server you do not use the access key registration system that I use in MC17. That works fine for me but I appreciate that I am not solving your problem - sorry
Paul