I'm guessing this is a late 90's frame, when the standards for disc brake mounts were being worked on. I did not see a brake mount on the seat stay, the preferred location. I suppose the mount is on the chain stay, which has led to many broken stays.
I cannot see where it is rubbing, looks like on the chainstay right? If it is only rubbing a little, why not put a 3 mm spacer in there to spread the dropouts a little like ciaran said? I know it is not as desireable a solution, but it is quick and easy, and the frame should tolerate that much deflection without an issue.