[Arokhaerr]

@Arokhaerr
Im going to guess you're on 5.4.5 and the "allow mature content" you're referring to is "Allow offensive content" 
"allow offensive content" was removed in 5.5.0, and the "age gate" setting is to enforce the age gate on some age-gated content when not signed it.
There are a few different ways content is age-gated by YouTube and some require login.
If you're on 5.5.0, signed in and still unable to view the video. Please provide at least the video id so I can attempt to reproduce, a debug_log (wiki) could be helpful too.

 https://youtu.be/rW-BArWIpD8 is one of the videos which wont play for me.

I am running the latest 5.5.0 on milhouse LE9

Location is germany. Video runs fine on android yt app an in browser.