Empathy should really give better error messages than “Network Error”. The Help->Debug window is not easy to decipher. I still have no idea what is causing it.
This time around I encountered the problem on an up-to-date Debian Testing installation. It affected my Facebook chat. The following command seems to fix it:
killall telepathy-butterfly telepathy-haze mission-control-5 telepathy-logger telepathy-salut telepathy-gabble empathy-chat empathy