Xcode compiles my App, but can't run it in the simulator

The solution for me was to delete everything Xcode has generated earlier:

• the App on the Simulator...if this is not possible because you cant reach the Homescreen, you can delete the App directly under ~/Library/Application Support/iPhoneSimulator/6.0/Applications
• delete the Derived Data in the Organizer under Projects or directly in ~/Library/Developer/Xcode/DerivedData
• clean the Build Folder by choosing "Product" in the MenuBar and click while you press the Alt-key on "Clean Build Folder"
• Restart Xcode

Have you tried closing down the simulator entirely, or failing that, reset the simulator and all its contents usually does the trick.