Refer to ClusterXL Administration Guide (Chapter 'Monitoring and Troubleshooting Gateway Clusters' - ClusterXL Error Messages - Sync Related Error Messages).
This message can appear when the machine is under extremely high load, when a Delta Sync update was permanently lost.
A Delta Sync update is considered to be permanently lost when it cannot be retransmitted because it is no longer in the transmit queue of the update originator.
This scenario does not mean that ClusterXL will malfunction, but rather that there is a potential problem.