Issue
I’m seeing a warning 'This node is standing by in case the primary node fails over' when I try to login to my Jenkins instance. However at other times login works fine.
Resolution
CloudBees High Availability (active/passive) Plugin provides CloudBees Jenkins users the ability to configure a secondary instance as a failover node. The failover node is in passive mode which means that it will not respond as if it were an active node. 'This node is standing by in case the primary node fails over' is the response from the secondary (passive) node in a High-availability (HA) pair. You will see this message if you attempt to access the secondary node or if you have a Load balancer (or reverse proxy) which is incorrectly setup to balance requests evenly between the primary and secondary nodes.
To correct this, do not direct requests to the secondary (passive) node. As well, do not use the secondary node responses as indication of the health status of your Jenkins instance. The secondary node is in standby mode and is designed to be ready (in case of failover) to take over from the primary node.
If instead of active/passive HA, you would like active/active HA, see: High Availability (active/active)