Symptom
Unable to connect to the Java VisualVM Plugins Center because of Server returned HTTP response code: 503 for URL: http://www.oracle.com/splash/java.net/maintenance/index.html
How to fix
The latest JDK points to the obsolete update URL
Create a new update URL
https://visualvm.github.io/uc/8u131/updates.xml.gz
cf.
VisualVM failing to find plugins/updates? Solving the 503 error with an updated URL
http://www.carehart.org/blog/client/index.cfm/2017/7/17/VisualVM_failing_to_find_plugins_updates_solution