User can't link Confluence pages to JIRA issues and gets a "You do not have permission to view this page" for Confluence pages in Issue Links

Users are unable to link Confluence pages to JIRA issues:

1. Open a JIRA issue.
2. Click 'More' > 'Link'.
3. Select 'Confluence Page' in the left menu.
4. Click 'search for a page' under the Page URL field.
5. Make sure that the 'Server' field is displaying the correct Confluence application.
6. All search results for ANY Confluence page come up empty.

In addition to that, when accessing JIRA issues that are linked with Confluence pages, users should be able to see the name of the page under the Issue Links section:

A few days ago this behaviour seems to have changed and now users are intermittently getting an "You do not have permission to view this page" message instead of the name of the page:

Clicking on the link still takes the user to the page, as expected.

Currently there are no workarounds to this situation.