When plugging my iPhone in and starting the Xcode organizer, a yellow circle next to the device show me that \"This device is busy or otherwise unusable by Xcode.\"
The Orga
With the help of somebody at the Apple Developer Forums I was able to fix the problem by removing the "/Developer/Platforms/iPhoneOS.platform/DeviceSupport/4.2.1 (8C148)" folder. Xcode then re-added the symbols automatically and the error disappeared.
For more information see this thread.