That's weird. I haven't seen any issues like this. Can you post the entire URL of the call that is being made to last.fm?
My bad, the problem is definitely on my end. I use foobar2000 under Wine and can't get any scrobbles now. I used a Windows VM before that which worked fine.
Do you happen to have an idea what could be needed to make it work? I have already tried installing .NET framework 2.0/3.0 but that didn't help.