The SQL Server Network Interface library could not register the Service Principal Name (SPN) Windows return code: 0x21c7, state: 15. Failure to register a SPN
  might cause integrated authentication to use NTLM instead of Kerberos.
  This is an informational message. Further action is only required if
  Kerberos authentication is required by authentication policies and if
  the SPN has not been manually registered.


I am getting above error whenever I restart my SQL service. I changed it from current service account to local account and was able to connect to SSMS from local machine, but if I am using current service account then this error is coming and in SSMS the error is : 


  The target principal name is incorrect. Cannot generate SSPI context.


The password is not changed for this service account.
You specifically get error "0x21c7, state: 15" when an attempt is made to register the SPN which already exists, but possibly under different account.

When you run SQL Service under local system account, an SPN is registered under the Computer Object in AD. You can confirm from below:

setspn -l ComputerName


In above results, If you see any SPNs for MSSQLSvc then they need to be dropped with below commands on AD (You need to have Domain Admin permissions for this) so that you can then register SPN under new Service account:

setspn -d MSSQLSvc/FQDN:XXXXXXXXXX ComputerName


Once the above is done, restart SQL Service under the Service Account adn it should successfully register the SPN under the new Service account and you will be able to confirm by running the below command:

setspn -l DomainName\LoginName


Reference: http://www.b-blog.info/en/changing-service-account-for-ms-sql-server-2012-and-the-following-issues.html

You could also use Microsoft Kerberos Configuration Manager for SQL Server to automatically detect and fix such SPN Issues:

https://blogs.msdn.microsoft.com/farukcelik/2013/05/21/new-tool-microsoft-kerberos-configuration-manager-for-sql-server-is-ready-to-resolve-your-kerberosconnectivity-issues/