I've deleted Last.fm Scrobbler from my Last.fm profile, now there's only JRiver software there, but nothing has changed(
I would consider uninstalling the last.fm scrobbler from your PC as well. Media Center does not need it as it scrobbles natively, and I can't help wondering if the other app is getting in the way, even though it has been disconnected from your profile.
There's no question that it works. We just need to figure out where yours has gone adrift.
Couple more questions:
Under "Tools > Options > Services > Last.fm", what do you have under "Configure filter for submission"?
Do other MC internet services work for you? Things like "Get movie / tv info", album art retrieval... that kind of stuff...
Do you have experience using the windows registry editor?
All questions/suggestions thus far are really just shots in the dark, which are frustrating for you when they miss, or just plain make no sense to you. I'm thinking we should reset and start again.