NUnit run fails the second time I open my just saved .nunit project

I'm going crazy with this NUnit GUI unexpected behaviour:

1) I run nunit-x86.exe, then I select File \ New Project \ name the .nunit project and Save , 2) select Project \ Add Assembly and select the .dll file related to my test asm , 3) NUnit gui load all the test classes tree correctly , 4) I run the X test class and everything goes fine , 5) I run the Y test class and everything goes fine ... I run others test classes or test methods and everything goes fine , 6) Then I save the project (File \ Save) and close the NUnit program , 7) I open the just saved .nunit project , 8) NUnit GUI program loads the test tree correctly but when I go to run a test method or a test class then the run fails. The error is an Exception

TestAsm.TestClass.TestMethod: System.BadImageFormatException : Could not load file or assembly 'ASM, Version= xxx, Culture= yyy, PublicKeyToken= zzz' or one of its dependencies. An attempt was made to load a program with an incorrect format.

What am I forgetting to check or set? Any hint will be appreciated.