When Connect with Kerberos SSO is checked, this string value must be set or all ticket requests will fail.
To generate the Kerberos Ticket for each user's logon, the SPN of your AS400/IBMi is needed.
Example: krbsvr400/systema.myco.com@MYCO.COM.
Note that you can check on any problems in the c:\programdata\[inventu|flynet]\fvterm\logs\Kerberos.log file.