Members of a cluster, after an upgrade of SEMS 3.3.2 to 3.4 and later, may have their replication process stop working. Cluster communication could be completely broken, or some cluster members could see others as active while others are unable to communicate. You can check this in the SEMS web console under System > Clustering and checking the Replication Status from each servers' web consoles.
Multiple scenarios can cause this.
STunnel service is not running. STunnel is responsible for delivering the replication information to the other cluster members securely.
The STunnel service will not run if there are issues with the /etc/stunnel/stunnel.conf file. The upgrade process might place duplicate Keyserver access entries into Keys > Keyservers. Remove all duplicates, and refresh the page to be certain they have not repopulated. If the error relates to the bad verification level, edit the stunnel.conf file and make sure "verify = 1" for the entry.
Stunnel can be restarted with the command "services stunnel restart". Restart the services or reboot the server once stunnel is corrected, and check status of replication. It should begin to replicate automatically.