Got a similar bug on the mai quest.It told me to defeat the Western empire and it show me -100 % advancement on the Q.The thing is,all Empire factions were defeated and the lords where vassals of other factions.Using Encyclopedia i found that the leader of the Western empire was Garios of the Comons.But his clan was a vassal of Battania.So i found him and convinced him(had no option to make peace,since we were at war) to join me.He did and the town he had,under battani rule, swiched to the Wester Empire.So now we have Easter Empire again in the game ,with 3 towns and castles and i am at peace with them.The leader is Chason of the Neretzes.I will go on defeating them and se if everything will get better.