@Jalmari K - It is very possible that your YouTube’s Visibility is set to Private, therefore when you are not signed in (incognito) it won’t play.

Update: Okay, scratch that thought - the message “unavailable” clearly does not mean “is private”.

Do you perhaps running a VPN with a browser extension component, so the videos will play when signed in, but won’t play when incognito (when the extension is likely disabled).

Are there location-based settings for your YouTube videos that will only play then in certain regions?

Some more troubleshooting on the topic:

https://geekdroids.com/youtube-video-unavailable/

@Jalmari K - Just doing a test of my own and not experiencing the same behaviour as you. I grabbed this random video from YouTube and put it in a board set to Anyone with the link → Can view.

https://miro.com/app/board/o9J_l1ezK5s=/

@Jalmari K - Can you play the video via an incognito window directly from YouTube.com?

@Robert Johnson Ah damnit, I double-checked this, and somehow it was still a private video.

Sorry for this, noob mistake All good now.

@Jalmari K - It happens to us all. It would have helped if the message showed when directly on YouTube’s site matched what they return for an embedded board.

If I go to youtube.com incognito:

But an embedded board returns:

There are several issues causing this problem. A few of the reasons why you might get such error include:

• There is a possibility that the video violates YouTube policy.
• The cache got corrupted and needs to be cleared.
• There may be a reason for the poor quality of the video.
• There may be an issue with the internet speed.

You can try out some options to avoid such situations of non-availability of your favorite videos. A few recommended options include:

• Try to reinstall your web browser.
• You can try a different browser to play the desired youtube video.
• Enable Flash and JavaScript.
• Deleting Cookies
• Update GPU drivers.

@warrensacko The issue was that the video was set to private, which the author clearly states in this reply: 

"Ah damnit, I double-checked this, and somehow it was still a private video.

Sorry for this, noob mistake All good now."