Why am I getting ibtool failed with exit code 255?

This happen to us and we clean cache, delete Derived data folder.. etc. nothing happen.

below steps made it fix,

1. open xib(nib) as Source Code (Right click on the xib Open As > Source Code from Xcode or open it using TextEdit)
2. remove this appearanceType="aqua"
3. save and open using Xcode

For more explain use this article, it help us to fix the issue.

I had something similar happen to me recently using Xcode 4.6 and iOS 6.1.

All I did was switch to a different device version (5.1) on the simulator and it ran. Switched back to 6.1 and it fixed itself.

Xcode can be unhelpful at times.