This is a conflict with your theme or with other plugins CSS.Please leave URL to this post to allow us find the conflicting code.

Posted by: Robert

This is a conflict with your theme or with other plugins CSS.Please leave URL to this post to allow us find the conflicting code.

Check this URL for example:  http://fotointresse.se/forum/teman-utmaningar/10-a-vertramp/

Thanks!

This comes from your theme style.css file. You can disable this for forum pages using CSS code below. Navigate to Dashboard > Forums > Settings > Styles admin page and put the code in Custom CSS Code textarea, delete all caches and do Ctrl+F5 on forum front:

#wpforo #wpforo-wrap .wpforo-post-content blockquote p:before {
     content: "";
     display:none;
}

Also please navigate to Dashboard > Forums > Dashboard and click on [Update Forums Statistic] and [Delete All Caches] buttons.