Busy with an Active Directory project in Visual Studio 2005 when I got this error:
System.Runtime.InteropServices.COMException (0x80005000): Unknown error (0x80005000)
Bit of research on Mr. Google and I discovered that my url was something like:
string connectionString = "ldap://blah.local/CN=xxxx Users
when I should have had:
string connectionString = "LDAP://blah.local/CN=xxxx Users
i.e. "LDAP" needs to be in caps.