Gerry Heidenreich

The authentication mechanism is unknown
I recently came across this issue when working with Active Directory in ASP.Net.  Turns out I needed to fully qualify my AD access username with the domain name.  When composing a DirectoryEntry([Domain], [Username], [Password]), precede the UserName with the Domain: 

DirectoryEntry de = new DirectoryEntry("LDAP://domainName", "domainName\gerryheidenreich", "myPassword");

I'm not sure yet why this web app is behaving different from others I've written that use AD, but this fixed the problem.