Cannot build in release mode (iOS in Xamarin Forms) with a valid certificate

Hi,

I try to build my application in release mode from a Mac with Xamarin Studio.

I get always this error: "Error while loading assembles: (MT0009) MTOUCHTASK"

NB. if I try to deploy my application on the emulator, everything is ok

here's the application details:

SDK version 7.1
Deployment Target 7.1
Linker: Link all assemblies
Xamarin Studio Version: 5.7.1
Xamarin.iOS 8.6.1.20
Xamarin.Forms 1.3.3

Any helps?

Thanks,

Alessio