It looks like this may be from a bug in Viper’s Video Quicktags. I was testing it out and this is the code VVQ is embedding the video with:
<span class="vvqbox vvqdailymotion" style="width:480px;height:221px;"><span id="vvq-923-dailymotion-1"><a href="https://www.dailymotion.com/videoxn9mou">https://www.dailymotion.com/videoxn9mou</a></span></span>
<script type="text/javascript">
swfobject.embedSWF("https://www.dailymotion.com/swf/xn9mou&colors=&autoPlay=0&related=0", "vvq-923-dailymotion-1", "480", "221", "10", vvqexpressinstall, vvqflashvars, vvqparams, vvqattributes);
</script>
The URL https://www.dailymotion.com/videoxn9mou is missing a slash and should actually be https://www.dailymotion.com/video/xn9mou. You can try visiting both URLs and see that the first cannot be found.
In vipers-video-quicktags.php try changing line 3040 (or somewhere in that area) to
return '<span class="vvqbox vvqdailymotion" style="width:' . $atts['width'] . 'px;height:' . $atts['height'] . 'px;"><span id="' . $objectid . '"><a href="https://www.dailymotion.com/video/' . $videoid . '">https://www.dailymotion.com/video/' . $videoid . '</a></span></span>';
This fixed the problem for me, so I’ll notify Viper007Bond about it.