Try add to admin > allowed iframes settings the domain: //coub.com should work with iframe and url version also.
The //coub.com is works 
<iframe src="//coub.com/embed/2p1n8u?muted=false&autostart=false&originalSize=false&startWithHD=false" allowfullscreen frameborder="0" width="640" height="360" allow="autoplay"></iframe>
The https://coub.com/view/2p1n8u also works well with this.