Problems with delegations i think
> check for duplicated\missed SPN (manual check on user\dekegation, ADSEDIT)
> check for SQL\service user & SQL ports if cluster cluster server or named istance
> check Login failed with error message 'NT AUTHORITY\ANONYMOUS LOGON'" on SQL and NAV Server...check on NAV Servers > impersonate rights on local groups
> check users on NAV & Mapped on SQL
Checks:
Try to delete and recreate kerberos tickets (launch KLIST purge on SQL Server, NAV Servers e try to connect)
If NAV 2009 try to check the system with "nav best practices analyzer" from partnersource
This is a tool that can be used to analyze "Login Failed" errors in SQL Server
blogs.msdn.com/.../sql-server-authentication-troubleshooter.aspx