Use Lotus Domino to send email in a .NET application

I have developed a solution to use Lotus Domino Object to send email in a .Net application. it works in Notes 6.5.4, but when I upgrade my Notes to 6.5.6, it doesn’t work, there’s following error when I try to creat a NotesSession:System.Runtime.InteropServices.SEHException: External component has thrown an exception

even though I reinstall Notes 6.5.4, it can’t work.

how to resolve this problem? thank you