Hi,
I have the same problem, since v14.0.130 (I upgraded because of the forced upgrade, I don't remember which version I was using before). OS is Win 7 x64.
Since then, tried with .137, .139 and now with .140 without success. I just tried a full (with library) uninstall/re-install of .140. Restored my license too. Looked for that VirtualStore/JRiver folder but the VirtualStore directory is empty.
Surprisingly enough, last.fm support was still enabled after the full re-install, and my username/password were still there.
Scrobbling does work when the last.fm plug-in is selected in the left panel of the main screen. But as soon as I return to Playing Now, or anywhere else, it stops working. The "Run the plug-in when MC starts" is always disabled when I go to the plug-in options screen (can't tell how many times I enabled it already!).
So... maybe there's a bug here... everything else works fine!
Thanks in advance for your help