event ID 4 security-kerberos what does this mean SBS 2011 network

Can anyone help me understand what this is saying?  Rob-PC exists now.  Joe-LPTP isn't on the network - joe's laptop maybe?  I searched Rob-PC's registry for joe but came up with nothing.  Both would be members of the domain, not servers themselves.

rob-pc doesn't have a mapping to a printer or share to joe-lptp either.

The Kerberos client received a KRB_AP_ERR_MODIFIED error from the server rob-pc$. The target name used was cifs/JOE-LPTP.contoso.local. This indicates that the target server failed to decrypt the ticket provided by the client. This can occur when the target server principal name (SPN) is registered on an account other than the account the target service is using. Please ensure that the target SPN is registered on, and only registered on, the account used by the server. This error can also happen when the target service is using a different password for the target service account than what the Kerberos Key Distribution Center (KDC) has for the target service account. Please ensure that the service on the server and the KDC are both updated to use the current password. If the server name is not fully qualified, and the target domain (contoso.LOCAL) is different from the client domain (contoso.LOCAL), check if there are identically named server accounts in these two domains, or use the fully-qualified name to identify the server