Opensubtitles decided to use API for logged in users only - so you need to be subscribed to them. Also, as per their forum, they mentioned that "the API servers are currently overloaded, due to many people staying at home" and that this was the reason they had to disable the API key publicly, to get it back to "normal parameters."
Unfortunately this is causing issues with loading subtitles automatically for us although the Videostream team is looking into ways that we can fix this so that automatic subtitles work again as soon as possible.
For where to find subtitles to download separately (if you don't want to auto-load from OpenSubs), try this website https://www.srtfiles.com/
From there (or if you have files with built-in subtitles), see below:
To enable auto-subtitles:
1. Go to Settings -> Subtitles -> Set up auto-loading subtitles
2. Add folders where your TV/Movies/Media is kept
3. Enable automatic display of subtitles
4. Now when you add videos, Videostream will automatically load and display any subtitles available for that media!
Hope this helps!