events manager "next month" and "previous month" buttons

Hi there,

I installed the plugIn "Events.Manager" (http://wordpress.org/extend/plugins/events-manager/).

When I hit the << or >> buttons for switching to the next or previous month the site disappears completelly and the calendar appears on a white background.
This happens only with the Atahualpa theme.

http://www.kidsarena.at/ka/

Is there something I can do?

greetings,
Gere