I had an error in the event log on every SharePoint 2010 farm server:

A Kerberos Error Message was received:
 on logon session 
 Client Time: 
 Server Time: 13:1:13.0000 3/17/2011 Z
 Error Code: 0x7  KDC_ERR_S_PRINCIPAL_UNKNOWN
 Extended Error: 0xc0000035 KLIN(0)
 Client Realm: 
 Client Name: 
 Server Realm: xxxxx.NET
 Server Name: HTTP/xxxxx.net
 Target Name: HTTP/xxxxx.net@xxxxx.NET
 Error Text: 
 File: 9
 Line: efb
 Error Data is in record data.

This happens after migration of the old MOSS 2007 farm to SharePoint 2010. – On the new farm I used new accounts for the application pools.

I created SPNs on the new Application Pool accounts for the web applications.

BUT I forgot to remove the same SPNs from the old Application Pool accounts!!! – This was the (big) mistake.

ADSIEdit let me create the same SPN on different accounts.

After removing the SPNs from the old Application Pool accounts everything works fine!

use SETSPN-Tool to find the accounts:

setspn Q HTTP/xxxx.net

You’ll get a list of all accounts that have the specified SPN “HTTP/xxxxx.net”