It seems the forum's current YouTube embedding relies on shockwave-flash. However, due to Flash's numerous security risks (hackers can use Flash exploits to gain control of your computer), more web browsers are disabling its use.
For example, I use Firefox and the YouTube shockwave-flash object is not rendered or displayed at all. In the case of the video linked in the OP, all I see is:
Here's an example:
In order to avoid the many security vulnerabilities of Flash, could the forum's YouTube embedding be changed to use an iframe instead?
There is a discussion on the phpBB forum here
https://www.phpbb.com/community/viewtop ... &t=2347051 which explains how to make this change to the BBcode.
This would have the added benefit of allowing the secure (https) YouTube URL to be used.