The support ticket mentioned over at Pidgin.im has a fix:
the liboscar.dll file is bad and needs replacement. They have uploaded a debug version of liboscar.dll in Pidgin ticket 6627, and it works with my setup. All you need to do is overwrite <Carrier>/liboscar.dll with the downloaded file.

The fix version of the dll is 1.6 megs, much larger than the current production version of the dll, because it was built with debugging.