I've heard some other problems with this recently, so I'm flagging this issue for more attention. Thanks for mentioning it.
As a workaround, paste your URL containg https://Youtu.be/ in the address bar of a browser, say Firefox and press Enter or Go. It will automatically expand to a larger URL with the Youtube and watch?v= keywords which is then acceptable in the community forum widget for a preview.
The short URL with youtu.be is produced by a 'URL Shortening Service' and is used to redirect to the full address of the video on YouTube. This is meant for use in places like twitter or an SMS where there is a limitation on the number of characters allowed. Other such sevices are goo.gl, tinyurl.com, and bit.ly.
You should not use these in the forum widget for previewing videos.