I've been developing an app for 1 or 2 weeks now and just yesterday I've updated my iPhone 5S to the iOS 8 GM. Everything worked fine and I could test on my device as well until I deleted the app from my phone and wanted to build again. The following error appeared:
Could not launch "My App"
process launch failed: Security
If you get this, the app has installed on your device. You have to tap the icon. It will ask you if you really want to run it. Say “
yes” and then
Build & Run again.
As from iOS 9, it is required to go to
Settings → General → DeviceManagement → DeveloperApp → Trust.