Symptoms

Administrators are unable to use the Universal Plugin Manager because of a firewall configuration preventing downloads. 

Cause

The Universal Plugin Manager makes calls to plugins.atlassian.com for metadata about the plugins installed. In order to pull down updates for the installed plugins, it contacts the various different repositories in which the plugin developers publish their plugins.

Workaround

Administrators can download plugins manually from Atlassian Marketplace, then upload them from the Confluence server (via Administration >> Plugins).

Resolution

1. As plugins.atlassian.com is a dynamic community of developers that undergoes constant improvement and change, it's very difficult to provide a static list of servers to whitelist for a static firewall. An initial configuration, to allow the plugins supported or hosted by Atlassian, is to open these repositories:

   https://*.atlassian.com 
   https://*.cloudfront.net

2. You may need to Configuring Web Proxy Support for Confluence.
   See the workaround above to understand about how to download plugins manually.