Found the answer. In my case I had my proxy settings defined in /etc/profile.d/proxy.sh so they were globally available. But I also had them defined in /var/lib/ambari-server/ambari-env.sh as -D options under JVM args according to Ambari installation instructions. While this didn't cause problems getting status for most components of my deployed cluster, it caused "503 - Service Unavailable" errors in Ambari Metrics and the Files View. Removing the -Dhttp.proxy settings from ambari-env.sh and re-starting solved the problem. If you are setting your proxy info such that your ambari user account can get them from the environment, don't assign them again in the JVM_ARGS for Ambari Server!
... View more