Similar error, since the system updated to 15.0.1
Code:
/* com.apple.ibtool.errors */
/Users/<user id>/Downloads/B4iBuildServer/UploadedProjects/<user id>/LaunchScreen.storyboard: error: Encountered an error communicating with IBAgent-iOS.
Failure Reason: IBAgent-iOS (-1) failed to launch
Recovery Suggestion: Please check Console.app for crash reports for "IBAgent-iOS" for further information.
Underlying Errors:
Description: Failed to launch IBAgent-iOS via CoreSimulator spawn
Failure Reason: Failed to spawn IBAgent-iOS on IBSimDeviceTypeiPad2x (4F455137-38C7-4AC3-826D-48A7CFA82769, (null), Shutdown)
Underlying Errors:
Description: Invalid device: The specified device is not available for spawning processes.
I don't have any device in the simulator list that has that UUID, and I'm not attempting to build for the simulator anyway, but to test on the app. It also won't build a release app, either.