In my experience Kerberos is really painful to configure properly, you will have to deal with all kinds of details regarding domain, accounts being part of the same domain etc.
If anything is misconfigured you'll always end up with about the same type of error. This means that this error is most likely caused by a configuration error. Unfortunately from the Mendix app it is impossible to get more details about this.
Kerberos doesn't offer any details either, some errors could show up in the system error log of the domain controller, but not always. I would recommend to start by looking there to see if an error might be shown.
Maybe we can help you validated the things you have configured, since this isn't always as straight forward as you might assume.
First of all is the Mendix app hosted in the same domain as the Mendix application. If the server is part of a different domain (by hosting in the cloud), or even internally in a different sub domain you'll have to perform additional configuration steps to allow Kerberos to respond to the Mx app.