The replica source %1:%2 requires Kerberos authentication but does not have a properly registered service principal name ...

The replica source %1:%2 requires Kerberos authentication but does not have a properly registered service principal name (SPN). 
An domain administrator can register the SPNs for the replica source by running the batch file that is in the data folder of the replica source. 
Alternatively, you can configure Kerberos authentication as optional; change the value of the attribute msDS-ReplAuthenticationMode from ADAM_REPL_AUTHENTICATION_MODE_MUTUAL_AUTH_REQUIRED to ADAM_REPL_AUTHENTICATION_MODE_NEGOTIATE on the Configuration partition of all instances in the forest. When msDS-ReplAuthenticationMode is set to ADAM_REPL_AUTHENTICATION_MODE_NEGOTIATE, replica partners can use NTLM for authentication if Kerberos is not available.