I have a free version of app. And there is a link to a full version in free app. The link works fine in iOS 6. But in iOS 7 it shows a blank page. Any help is appreciated!
Try this one, it's the new syntax with iOS 7 and replace APP_ID by your application's AppID.
itms-apps://itunes.apple.com/app/idAPP_ID
You can refer to this link and this one for more information and discussion about that.