I just came across a good article by Keith Brown on a common trap when
setting up

SQL Server with kerberos delegation
. As he noted, the port number for the
Sql Server Service should be specified when setting the service principal name

Delegation is a powerful feature of Active Directory but can be difficult to
setup. It can be used to allow SQL Server to impersonate the user when
connecting to other services: such as BULK LOAD connecting to a network file
server. Or for middle tier applications to impersonate users on other servers. A

explanation of delegation can be found here

Microsoft’s whitepaper on

troubleshooting kerberos delegation
has more detail on setting up SQL
Server, including linking servers with delegation.