I was having issues with the rendering of responses when videos were posted. It seems that this is due to the insert media and link buttons on the WYSIWYG toolbar, I think, because Simple Press has native embed ability, i.e. post a youtube link it embeds naturall, post a html link it turns into a hyperlink. I think when you use the TinyMCE plugin, that these two buttons means you have two bits of code trying to do the same thing so they compete with each other and make a post with links or videos unstable.
I can't provide screen shots because I fixed the problem.
SimplePress has native ability to turn links to videos into embedded videos and to turn links into hyperlinks. When you try to use the media insert buttons on TinyMCE, the post becomes unstable, likely because those functions compete with the native ability of the forum. I notice that you have those buttons disabled on this forum.
The plugin needs a conditional piece of code that turns off the native embed functions when the TinyMCE media / hyperlink buttons are used.