Merge branch 'docs/geo-404-secondary-node' into 'master'

Docs - Geo admin area showing 404 error for a secondary node

See merge request gitlab-org/gitlab!20513

doc/administration/geo/replication/troubleshooting.md

@@ -701,3 +701,17 @@ See ["Foreign Data Wrapper (FDW) is not configured" error?](#foreign-data-wrappe
 
 This can be caused by orphaned records in the project registry. You can clear them
 [using a Rake task](../../../administration/raketasks/geo.md#remove-orphaned-project-registries).
+
+### Geo Admin Area returns 404 error for a secondary node
+
+Sometimes `sudo gitlab-rake gitlab:geo:check` indicates that the **secondary** node is
+healthy, but a 404 error for the **secondary** node is returned in the Geo Admin Area on
+the **primary** node.
+
+To resolve this issue:
+
+- Try restarting the **secondary** using `sudo gitlab-ctl restart`.
+- Check `/var/log/gitlab/gitlab-rails/geo.log` to see if the **secondary** node is
+  using IPv6 to send its status to the **primary** node. If it is, add an entry to
+  the **primary** node using IPv4 in the `/etc/hosts` file. Alternatively, you should
+  [enable IPv6 on the primary node](https://docs.gitlab.com/omnibus/settings/nginx.html#setting-the-nginx-listen-address-or-addresses).