It does look like you might have a timeout like @Christian Patterer mentioned.
On another question thread where this is marked solved the accepted answer is,
"This means the Jira server you have set up in the application link is not responding to the query. Go to admin -> application links and check the link is correct. If it is, then check that Jira is currently running on that url, and then that your Confluence has a route across your network over to the Jira."
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