I see... yes this is a bug. It appears to be a case sensativity issue. We tried to update every page and link to all lower case, but it appears this one got by.
You can fix it on the page: webassist/plugins/catalog/popularbox.php
on line 43 update Products_Detail.php to products_detail.php
I apologize for the inconvenience. We'll get this into the the next dot release.