Brand new Jenkins instance is behind corporate proxy. Test URL passed, but checking plugins ends in "There were errors checking the update sites: FileNotFoundException: http://updates.jenkins.io/update-center.json?id=default&version=2.176.3", see screenshots.
This URL is normally available in browser. When "Use browser for metadata download" ticked, I'm able to browse plugins but I can't download anything. I tried update mirrors (mirror.serverion.com) as update URL but still nothing.