If you see an error like on the screenshot below then it most likely means the Firewall is blocking connection from Jira nodeA to nodeB and vise versa on port 8081 (your configuration may have a different Jira node port number in the URL):
3. Open your Firewall configuration and allow 8081 port (or 8080) depending on what you see on your Maintenance Page for communication between cluster nodes.
That should be it.
Simply refresh the Maintenance Page to check if the connection is successful now.