Somewhere in its code the forum is inserting a double slash in the URL.
Try a forum search (for "hello" - whatever really) and click on one of the search results. You'll get something like:http://forum.arduino.cc//index.php?topic=197001.0
Now, Chrome (and I suspect many other desktop browsers) forgives the // error and displays the page anyway. But some systems are not so forgiving.
For example, see my screenshot of an emailed new topic notification as it appears on my Android mobile. It links only to the root, stopping at the incorrect //.
Can the erroneous duplication be found and weeded out?