After navigating to http://<jenkins-url>/pluginManager/advanced and clicking "Submit" under the Update Site section, I noticed the available plugins list was empty. The search functionality works but prevents any plugins listed from being added.

Navigating to the provided URL, https://updates.jenkins.io/current/update-center.json, in Firefox cause the following error to be displayed under the JSON tab:

SyntaxError: JSON.parse: unexpected character at line 1 column 1 of the JSON data

The GET request to /available appears to have the list of plugins. Just looks like the add buttons are not rendering.