Scrum and UCD

Hi,
I'm writing my Master's Thesis at the moment and in my Thesis I'm trying to find the biggest problems between the two methods and then to integrate Scrum and UCD and create new user-centered Scrum.
My Thesis is based on literature about integrating user-centered design and agile methods. My Thesis is almost ready and at the moment I am starting to introduce my process to one software project where I am the UE designer.

I would like hear from You, what kind of actual troubles and problems You have had when using both Scrum and User-Centered Design?
And what advantages has the use of both methods brought to the project?

Waiting for your opinions,
Linda.