NCover is breaking NUnit tests

Hi,

I have this new problem:
Some of the new unit tests that we wrote, are been executed alright with nunit (gui and\or console) but fail when nunit is executed through NCover.
Most of the error contain this chain of information:
System.TypeInitializationException : Exception from HRESULT: 0x80131534.
or-
--> System.NullReferenceException : Object reference not set to an instance of an object.
System.TypeInitializationException : Exception from HRESULT: 0x80131534. TearDown : System.Reflection.TargetInvocationException : Exception has been thrown by the target of an invocation. ----> System.NullReferenceException : Object reference not set to an instance of an object.

did someone encountered that problem? What is the exception number means?
need help..

* Also I'm using NCover 1.5.4 so it's got nothing to do with the 1.5.5 bugs that were posted here lately. I checked this error in (1) NANt task (2) normal cli execution (3) NCoverExplorer new window (Ncover Runner)

Thanks,
M