I discovered a minto Bug in the event_calendat module in the 2.4 trunk.
When using $LiveCalendarWidget the 'CurrentMonthLink' is generated wrong, so that clicking it results in a 404 error.
The problem is, that the link lacks the 'view/' in the URL (e.g. mydomain.com/calendar/view/201006)
To make the widget work correctly under SS 2.4 just use this simple fix:
Replace line 249 in event_calendar/code/CalendarUI.class.php with this code
'CurrentMonthLink' => $this->calendar->Link('view')."/".$this->start_date->format('Ym'),