Unity iOS - "Assertion at reflection.c:6869" only in Development Build

Hi guys!

We have a C++ lib we use for our Android & iOS unity app. We've encountered a problem that only occurs on iOS platform.

Here's the scenario: we have a callback function in Unity-CSharp. We give the pointer of that function to the C++ lib, that will call it at certain points of the code. The signature of the callback is like this:

 private delegate void CallbackFromNative(int int1, int int2, int pNumberOfParams, [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPStr, SizeParamIndex = 2)]string[] pParams);
 
 [MonoPInvokeCallback(typeof(CallbackFromNative))]
 private static void Callback(int int1, int int2, int numberOfParams, [MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.LPStr, SizeParamIndex = 2)]string[] pParams)    
 { ...}

Calling that callback is successful most of the time (95%). We found 2 points (so far) when calling results an assertion. This is all we get in XCode:

 Assertion at reflection.c:6869, condition `idx != -1' not met

Not too helpful, isn't it...

So once again: on Android there is no error with the exact same code; it only occurs on iOS, and even on iOS it only occurs some times, not every time we call the callback.

EDIT: another interesting thing: We recognised that it only occurs when "Development Build" is checked in in Unity3D's Building settings

Unity3D version: 4.3.4f1, Xcode version: 5.1 (5B130a) (with Xcode 5 it was the same)

Any idea, hint and help would be appreciated! Thanks in advance!