I can only assume its a bug with the courier. I was doing the White Phial quest line. After i completed the quest, I waited around for the courier to show up and give me the letter from Quintus. Subsequently he has yet to show, and its be days(game time) and I have tried shouting in various cities to no avail. As well, this may in directly relate to the courier. While waiting for the courier for Repairing the white phial, I started Arniels Endeavour and was completely that quest. I got to part 4 and Arniel disappeared, now i am unable to initiate this part. I know that there is a courier that makes an appearance in this quest. But seeing as how I am unable to proceed, I cannot confirm a courier issue.