I had that problem once, or something similar... on another Quest right after doing the Tutorial Quest all the way through.
Everytime I had the questgiver as an Alias, the questgiver would stop having the dialogue to the quest.
Where I went wrong was on the Quest Stages Tab. Just to the right of the 'log entry' area you will see where the 'script fragment' area is. Just below that is the 'Properties' area. In the properties I did not reference the Alias of the Questgiver to the Actor as the value of that property.
Once I referenced the Actor to the Alias then it all worked fine.
I really hope this helps. I know how it is searching for one thing in a heap of information.