I have a Tubus Tara on my 2007 Trek 520 and had a similar problem. I figured out it was being caused by a small defect on the right eyelet of the fork. The eyelet was angled forward slightly, and this caused the arm of the rack to be pushed forward and put the rack off center.

I fixed the problem by enlarging the hole on the Tara rack to accommodate the angled eyelet. It's not perfect, but it's certainly better.

Good luck!