To Tigerlilly:

I reckon you probably need to remove '-' from the following code in handleAction method:
$patterns[] = '/^(19|20)\d\d$/';
$patterns[] = '/^(19|20)\d\d-(0[1-9]|1[012])$/';
$patterns[] = '/^(19|20)\d\d-(0[1-9]|1[012])-(0[1-9]|[12][0-9]|3[01])$/';

After removing '-', the code looks like this.

$patterns[] = '/^(19|20)\d\d$/';
$patterns[] = '/^(19|20)\d\d(0[1-9]|1[012])$/';
$patterns[] = '/^(19|20)\d\d(0[1-9]|1[012])(0[1-9]|[12][0-9]|3[01])$/';

The files of Canlendar.php and CalendarEvent.php have been attached.

Sorry for being stupid, but where can I find the 2.4 Version of the EventCalendar?
Didn´t find it in the Module section (http://www.silverstripe.org/event-calendar/) nor on the SVN Server (http://bluehousegroup.svn.beanstalkapp.com/modules/trunk/event_calendar).