What you should do if you're stuck is, in a shell:
ibtool --warnings --errors --notices yourfile.xib > alerts.plist
which will crash, but now you have the crashlog, in my case it was an issue with simulated metrics:
localhost:Classes me$ ibtool --warnings --errors --notices CodeViewController.xib > alerts.plist
2014-04-25 17:40:48.625 ibtoold[11691:507] [MT] DVTAssertions: ASSERTION FAILURE in /SourceCache/IDEInterfaceBuilderCocoaTouch/IDEInterfaceBuilderCocoaTouch-5038/InterfaceBuilder/WidgetIntegration/SimulatedMetrics/IBUISimulatedSizeMetrics.m:33
Details: (initialTargetRuntime) should not be nil.
Object:
Method: -initWithTargetRuntime:
Thread: {name = (null), num = 1}
Hints: None
So in my case I opened the xib as source via context menu and removed the metrics objects. It depends on the error what to remove or change. Try ibtool again and if it doesn't crash you're good.
Background:
The reason for this error is generic, it can always happen with different specs changing from iOS xcode version to version, mostly if you haven't checked your projects periodically with xcode updates.