Issue

• All of my Jenkins masters except for one are connecting to CloudBees Jenkins Operations Center
• Several of my Jenkins masters connect to CJOC.
• No logs on CJOC as to why a connection can not be established.

Environment

• CloudBees Jenkins Operations Center
• CloudBees Jenkins Enterprise
• Jenkins

Resolution

One reason why CJE will not be able to connect to CJOC is because of the proxy configurations in the plugins page.
On the Client Master which is not able to connect to CJOC go to Manage Jenkins > Manage Plugins > Advanced tab.

If there is proxy configuration defined in this section, then please make sure to set the no host proxy for the CJOC url.
The client master will use this proxy configuration to connect to CJOC initially.
If this is configured, it will use the proxy (potentially leaving the network), and try to run HTTP GET operation.
By setting the no proxy host option, this would make sure that for connecting to CJOC it does not use the proxy configurations.