Something have changed with the forum engine for about a week ago with this subject.

A week ago:
I was able to watch embedded video with Nokia default web browser for my mobile phone (BrowserNG/7.2.8.10).
But this browser not quite user friendly (actually it is quite boring to use it) so I also use Opera Mobile 11.10.
Opera is almost perfect in user friendliness but can't play embedded video it was even impossible to guess whether there were a video embedded in a forum's post or not.
Nevertheless Opera Mobile is able to lunch default phone's player for youtube site and video can be played if accessed directly at youtube html page (this is why I've asked about an alternative link).

This day:
Both browsers shows up a black box where a video is embedded but neither of them can play or launch the player to play it or redirect to youtube source page.

The good point is: now when using Opera Mobile I am aware that there is a video.
The bad point is: It became absolutely impossible to watch video (or even to guess a title) with mobile phone, neither of the browsers can play it now.