I have an UIActivityViewController that is shown on an UIViewController (named viewCon here) :
UIActivityViewController
UIViewController
// items contains text and/or image
I guess this is an iOS7 bug. It appears with some special sourceRect value. You can fix it by set another sourceRect, for example you can try:
sourceRect
[activityView popoverPresentationController].sourceRect=CGRectMake( 0,200,768,20);