docs: keep Apache's /error/ pages out of the proxy so 401 prompts for a password

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

docs/lab/prod/on-router/README.md

@@ -70,6 +70,16 @@ API and POSTs through the proxy, and events arriving within 50 ms):
             Require valid-user
         </Location>
 
+        # Apache's own error pages live under /error/ on servers with the
+        # multi-language error documents enabled. They must be served by
+        # Apache, not proxied, and must not themselves demand the password.
+        # Otherwise the 401 becomes ferrix's empty 404: a blank page and no
+        # password prompt in any browser without saved credentials.
+        <Location "/error/">
+            Require all granted
+        </Location>
+        ProxyPass        "/error/" "!"
+
         ProxyPreserveHost On
         ProxyPass        "/" "http://127.0.0.1:8080/"
         ProxyPassReverse "/" "http://127.0.0.1:8080/"