From the blog "Emby Blog"

An updated version of the Emby Open Subtitles plugin is currently rolling out. We have switched from opensubtitles.org to the newer opensubtitles.com This means you will need to create an account on opensubtitles.com and configure that in Emby Server settings. Until you do that, you won't get any subtitle downloads. Stay tuned for more developments on subtitle downloading !