The Best Answer extension probably adds information to the page in a format that Google doesn't like.
As the text in the screenshot says, it doesn't look like it will prevent your site from appearing in search results, just that the advanced features related to Google question/answer template will not be applied.
To fix the issue, someone who knows how those search engine tags work should take a look at the extension and the site markup to figure out what part of the extension is responsible for the content that Google doesn't like. I doubt it's an error from the forum owner, it's probably either an error in the extension or a specific situation that was not taken into account when the extension was written. Or maybe Google updated their requirements since then and the extension no longer meets them. It's difficult to say, I'm not familiar with that Google feature.