I got the same bug (did quests for Knights and Dozens and now I cant get the invitation) for a week or so I am stuck. Since I dont want to cheat or mess with any code like it was mentioned earlier here in this thread, I am waiting for a week or so for patches. 1.03 didnt solve the problem but one developer said 1.04 would be it ( http://forums.obsidian.net/topic/72793-major-plot-progression-bug-act-2-the-duc-invitation/page-2 ). Unfortunatelly I couldn't progress wenn I downloaded its Beta.
So, any words from the developers? You made a really great game but some of us players are stuck for more than a week now