Symptoms

1. In the application link configuration pane, there is a warning that 'Application JIRA seems to be offline'
2. When adding a link within a confluence page, it shows the following error.  
    

   'Not Found<br/>class java.lang.RuntimeException'

    

3. The following appears in the atlassian-confluence.log:

   ERROR [http-8090-13] [confluence.extra.jira.JiraIssuesServlet] doGet Unexpected Exception, could not retrieve JIRA issues: Not Found
   – url: /plugins/servlet/issue-retriever | userName: liumiao
   	java.lang.RuntimeException: Not Found
   	at com.atlassian.confluence.extra.jira.DefaultJiraIssuesManager.checkForErrors(DefaultJiraIssuesManager.java:232)
   	at com.atlassian.confluence.extra.jira.DefaultJiraIssuesManager.access$000(DefaultJiraIssuesManager.java:45)
   	at com.atlassian.confluence.extra.jira.DefaultJiraIssuesManager$1.handle(DefaultJiraIssuesManager.java:116)
   	at com.atlassian.applinks.core.auth.ApplicationLinkRequestAdaptor$1.handle(ApplicationLinkRequestAdaptor.java:88)
   	...

Diagnosis

Try to configure the Application Link using various addresses of the JIRA Server; such as the domain or IP address of the server on local, private and public.

Cause

Some redirection, such as and poorly configured network, makes the JIRA server unable to be discovered. 

Resolution

1. If there's a proxy configuration in catalina.properties, remove them as it affects the network traffic
2. Make sure the network configuration such as /etc/hosts or /etc/resolv.conf is configured properly and the domain resolves to the correct IP address.