Application link health check fails with spurious network error

Summary

Health Check is returning an error when connecting JIRA 7.4.0 to Confluence 5.9.5 although the application link is working according to the 2 following points:

• in JIRA Admin > Applications > Application Links, the status of the Application Link is "Connected"
  

• when trying to link a Confluence page to a JIRA issue from the Link dialog, the Confluence page is properly connected
  

Steps to Reproduce

1. Install JIRA 7.4.0
2. Install Confluence 5.9.5
3. Create an Application link to Confluence from the JIRA UI
4. Go to JIRA Admin > System > Support Tools
5. Click on "Services" to expand it
6. Click on "Application Links" to expand it

Expected Results

The result should be "Your application links are all working properly", as it's the case when linking JIRA to Confluence 6.3.1.

Actual Results

The result says "One of your application links is reporting "Network error".", although the application link is working and it is possible to link Confluence pages to JIRA issues:

Also, when navigating to any other JIRA page, a pop-up shows at the top-right corner with a similar error:

Lastly, we can see the following warning in the JIRA logs:

2017-07-13 12:25:16,020 HealthCheck:thread-6 WARN      [c.a.applinks.core.DefaultApplinkStatusService] Unrecognized error while attempting to retrieve status of Application Link 'df6813a6-ecd0-3c00-8c59-1673445a2fc4'

NOTE

It should be clearer which AppLinks versions are compatible with each other:

• JIRA 7.4.0 is using AppLinks 5.3.2 while Confluence 5.9.5 is using AppLinks 5.0.4, and in the Application links version matrix documentation, it is not clear whether 5.3.2 is supposed to be compatible with 5.0.4.

If there is any incompatibility issue, it should be clearly specified in this documentation.

Workaround

It's reported that upgrading Confluence to the latest version may help, but this is not guaranteed as there's also a report that JIRA 7.4.4 shows the same warning connecting to Confluence 6.3.4.

 If there's no actual applinks problem, the warning may just be ignored or dismissed.