CLR Hosting and private assembly issue

I'm loading an assembly AA through CLR Hosting. It works...

In this Assembly, I add a reference to another custom assembly XX (A minimal class "Hello world") and instantiate this XX within a method in AA.

But now running again AA with CLR Hosting raises the error :

0x80131604 : Uncaught exception thrown by method called through Reflection.

Note : If I register XX in GAC, everything works ! I tried too a dynamic loading like

Assembly am = Assembly.Load("XX");

it works too...

An idea ?