The system property "http.proxyPort" (passed on the command line as "-Dhttp.proxyPort") is ignored. Whatever its value, confluence only try and connect to port 80 of the specified proxy. In consequence, RSS feed as well as plugin management don't work.

After a short investigation I found a bug in the "com.atlassian.confluence.util.http.httpclient.HttpClientHttpRetrievalService" class:

            int port = 80;
            try
            {
            	Integer.parseInt(System.getProperty("http.proxyPort", "80"));
            }
            catch (NumberFormatException e)
            {
                log.warn("System property 'http.proxyPort' is not a number. Defaulting to 80.");
            }

            client.getHostConfiguration().setProxy(host, port);

The return value of parseInt should be stored in the "port" local variable.