Date Picker Problem

When i add a new (the first) Event, the EM tells me that the Date Format is not correct and i have to use the Datepicker. My Problem is: i use the Datepicker the whole Time. How can i fix that?
(the Picker Result is that: “27/06/2013 – 12/07/2013” for Reservations)

I use WordPress 3.5.1, Twentytwelve, german Translation
Plugins:
Contact Form 7
Google XML Sitemaps
Revolution Slider
WP Bakery Visual Composer
URL: http://www.http://plus-seminare.info/veranstaltungen/

Thanks for your help!

http://ww.wp.xz.cn/extend/plugins/events-manager/