Xcode Storyboard Warning not providing context: Unsupported Configuration of constraint attributes

Open build log and check the warning by expanding the details by tapping the small box icon at right side of the warning. You should find something like this at the end:

/* com.apple.ibtool.document.warnings */ /Users/username/git/folder/project/Collection/Collection.storyboard:2f5-sJ-euT: warning: Unsupported configuration of constraint attributes. This may produce unexpected results at runtime before Xcode 5.1

Now open that storyboard as Source Code and search for the ID mentioned in warning. In this case it's 2f5-sJ-euT. You should find something like this:

<constraint firstAttribute="centerX" secondItem="ReE-6A-euY"
    secondAttribute="centerX" id="2f5-sJ-euT"/>

Now you can check what's the problem. To help that you could e.g. check where is this constraint by looking at possible reuseIdentifier of the containing object. Makes it easier to find it, when you open the storyboard in Storyboard format.

In my case the warning was about a constraint that wasn't needed any more and I just removed it. And tested that the app still worked like it was supposed to, of course.