Problem scenario: You are in the Jenkins master web UI and are trying to configure a Jenkins node (previously known as a slave) instance. You entered the correct hostname, username and password. You get this error:
"ERROR: Unexpected error in launching a slave. This is probably a bug in Jenkins.
java.lang.IllegalStateException: Connection is not established!"
Solution: Instead of using the hostname of the Jenkins node (previously known as a slave) server, use its IP address.
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Problem scenario: You installed a new Jenkins instance. When you go to the IP address over port 8080 in a web browser, you get a blank white screen.
Solution: Go to Internet Explorer's tools (click the gear symbol if that is the version you have) and go to Options. Go to the Security tab. At the top, click the "Trusted Sites" icon. Click "Sites." Enter "http://x.x.x.x" with no quotes and replace x.x.x.x with the IP address of Jenkins. Click Add. Click Close. Click Ok. Now refresh your web browser. You should be able to go to Jenkins' UI.