The layout is totally ugly. The theme we use is Lumix. See screenshot on http://xanthias.de/galerie/wp-calendar.png
This issue is because of CSS conflict with your theme.
Please send the link to that page and we will help you in fixing that CSS issue.
PS It do not depend from the version of WordPress.
Thank you for your reply. Meanwhile I switched to Contact Form 7 and the Contact Form 7 Calendar Plug-In. That works great.
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