well, i don't know if this is ngui fault but here it is the stack calls.
A first chance exception of type 'System.Runtime.InteropServices.SEHException' occurred in UnityEngineProxy.DLL
Exception: External component has thrown an exception.
Type: System.Runtime.InteropServices.SEHException
Module: UnityEngineProxy
InnerException: <No Data>
AdditionalInfo:<No Data>
at UnityEngineProxy.InternalCalls.Component_CUSTOM_InternalGetTransform(Object self)
at UnityEngine.Component.InternalGetTransform()
at UnityEngine.Component.get_transform()
at UIRect.get_cachedTransform()
at UIPanel.UpdateTransformMatrix()
at UIPanel.UpdateSelf()
at UIPanel.LateUpdate()
at UIPanel.UnityFastInvoke_LateUpdate()
This happens on change to a complex scene with multiple panels (all siblings) and occurres only after adding an interval loop that does Microsoft.Xna.Framework.FrameworkDispatcher.Update(); every 5 seconds (this is needed to don't make the app crash when wp8 music player is playing)... so really, i don't know if this is related to ngui.
I will try to do more research around the xna framework