Details are in this thread:
But essentially you want to disable the preview window when trying to post a video link, otherwise when it creates the preview, it tricks the board software into thinking you’re trying to add the video as an attachment rather than provide a link.
Once the video preview has been created, whether by accident or on purpose, it’s “tainted” and it won’t let you post it anymore. So you trick it by making it think it’s another video and add a #1 to the end of the URL, which makes it look like a different link but still goes to the same video. And if the link has already been done with #1, you can do it as #2, #3, and so on.
That’s what I did in my post; I put the same link that you did but added #1 to the end. I also made sure to disable the preview pane so that it didn’t create the preview before I submitted the post.
This also lets you edit a post with a video in it. Let’s say you successfully post the video but realize you made a mistake. If you edit it and try to save it you’ll also get the error about embedding media. So before you save it, just do the same trick; add a #1, or increment the existing number to the next one.