.Net's Directory Services throws a strange exception

Question

I have a small C# solution used to check users credentials. It works fine for two of my teammates, but on my PC I get an exception.

The relevant code:

Answer 1

I had this problem too using IIS Express and VS 2010. What fixed it for me was a comment on another thread.

Validate a username and password against Active Directory?

but i'll save you the click and search... :) Just add ContextOpations.Negotiate to you Validate Credentials call like below.

bool valid = context.ValidateCredentials(user, pass, ***ContextOptions.Negotiate***);