Anonymous users prompted to login when searching Confluence

Summary

Anonymous users cannot search Confluence through the top right search field. When an anonymous user clicks on the top right search box, the following error is quickly displayed and Confluence redirects user to the login page:

Looks like we lost your session.
We'll hold on tighter next time, promise.

Environment

• Confluence 5.9.1
• Chrome, Firefox, Safari (latest)

Steps to Reproduce

1. Enable anonymous access
2. Go to the main dashboard in Confluence
3. Without logging in, click on the top right Search box

Expected Results

Cursor is shown and you can enter search text.

Actual Results

No errors thrown in the atlassian-confluence.log. The following warning flashes quickly and user is redirected to the login page:

Workaround

Go to your search URL directly:
http://<confluence_url>/confluence/dosearchsite.action?