I came across this very strange error today while working with the Enterprise Library Security Block on an ASP.Net application:

System.Data.SqlClient.SqlException: Cannot initialize SSPI package.

I'd never come across this error before and was a bit confused as everything seemed OK on the SQL server and the permissions were set correctly (as far as I could make out at lease)

After a bit of searching I found the KB http://support.microsoft.com/default.aspx?scid=kb;EN-US;q253577 which states that cause for this error is that you have a custom application dynamic-link library (DLL) file that is named Security.dll - which is exactly the name of my dll.

So I renamed the dll but got the same error when I ran the app again - only to realise that the rebuild still left the old Security.dll in the bin directory, after deleting the dll everything worked fine.