We, the Microsoft Dynamics NAV Support Team in the Customer Service and Support (CSS) department, see from time to time cases asking for the Service Principal Names (SPN) that are required for Dynamics NAV Web Services (WS) to run properly on a three-tier environment with Kerberos enabled.
I had a customer that had an issue with setting up Windows Logins in NAV 2009 R2. It appears that there were some issues in Active Directory when setting up new users, the users were then added to NAV and then a change was made in Active Directory which prevented the user from logging into NAV. I deleted the Windows Login in NAV, Synchronized All Logins, tried to re-create the Windows Login in NAV with no luck. I then turned to SQL Server.