I configured pms to run on my mac mini, connected an external hd where I have all media files. Configured the agents Settings > Agents > Movies > Freebase > 'OpenSubtitles.org' (as first) and then 'The Movie Database'. Also in Settings > General > Enable Plex Media Server debug logging. In the logs I see something like this:

{
"type": "ajax",
"url": "http://127.0.0.1:32400/system/agents/attribution?identifier=com.plexapp.agents.opensubtitles",
"status": 404,
"headers": {
"X-Plex-Product": "Plex Web",
"X-Plex-Version": "2.6.1",
"X-Plex-Client-Identifier": "5hdoiip3qalwhfr",
"X-Plex-Platform": "Chrome",
"X-Plex-Platform-Version": "50.0",
"X-Plex-Device": "OSX",
"X-Plex-Device-Name": "Plex Web (Chrome)",
"X-Plex-Token": "TOKEN",
"Accept-Language": "en"
},
"statusText": "Not Found",
"time": "2016-05-28T08:52:49.733Z"

},

When entered "http://127.0.0.1:32400/system/agents/attribution?identifier=com.plexapp.agents.opensubtitles" on the server browser, I see a blank page