I’ve come across the same issue. Although I’m not exactly sure what caused the bug (I updated wordpress and the calendar plugin around the same time). I noticed that the descriptions with dollar amounts in them ($10 etc.) would not display on the mouse-hover popup.
The fix required a slightly different modification:
$details = '<span class="calnk"><a href="'.$linky.'" '.$style.'>' . stripslashes($event->event_title) . '<span '.$style.'>' . $header_details . '' . stripslashes(str_replace('$','$',$event->event_desc)) . '</span></a></span>';
Same thing other than using '$' rather than '\$' . Somehow the escaped dollar symbol '\$' didn’t want to take. Replacing the dollar symbol with the html entity code solves it.
