I am having this same problem in my installation (on AWS).
ErrorException: stream_socket_client(): unable to connect to x.xxx.xxx.xxx:80 (Permission denied)
The IP is the same for Omeka S and MediaWiki. I can navigate to http://x.xxx.xxx.xxx: to see my Omeka site and I can navigate to http://x.xxx.xxx.xxx:/mediawiki/api.php to see my MediWiki. I can curl the MediaWiki address from both the server and my local Macintosh.
I am not using a proxy and everything is on the usual port number.
The access_log shows the request coming back with a status of 200, oddly.