JSM Knowledge base error - Restrict to articles with labels blocks all pages

Issue Summary

When using Jira Service Management Knowledge base on version 5.11 or later, an error related to "Restrict to articles with labels" blocks all pages from being displayed.

Environment:

• Jira Service Management 5.11 or later (also tested on JSM 5.14).
  The issue is not seen on JSM 5.4.latest
• For the tests we used Confluence 8.5.5 and 8.8.0

This is reproducible on Data Center: yes

Steps to Reproduce

1. App link JSM 5.11+ to a Confluence
2. Create a JSM project and setup a Knowledge Base
3. In "Auto-search on request forms" restrict any request form to use a specific label in the "Restrict to articles with labels"
4. Go to the customer portal and in the request form set previously type a word that you know that should return a result.

Expected Results

Return all results as expected.

Actual Results

No result is displayed. The images below show the behavior:

Without adding any label to the "Restrict to articles with labels", the KB search works
When adding any label to the "Restrict to articles with labels", the KB search does not show any result

Workaround

You can use the dark feature "sd.use.confluence.v2.search.disabled" to use the previous class of knowledge base search, which does not present this error. For that, go to the dark feature page <jira-url>/secure/admin/SiteDarkFeatures!default.jspa and enable that flag. 

Alternatively, if suitable, you can remove all label restrictions.