Light spot dead-center with extension tubes

I bought a cheap -chinese-made- set of metallic extension tubes that I used with a Nikon D60 without having any issue. Today am trying to use them with a Nikon D7000 but for some reason whenever I use them -no matter which combination I choose-, there's always a light-spot dead-center on the photos, just like below:

Anyboody has any idea as to why is this happening?

This is particularly noticeable whenever I use my macro flash.

Thanks for looking!