false-positive SPAM

Hello,

I get an error message with an orange border even though I don’t use Akismet. I use WP-Super-Cache. I have the appropriate garbage collection (clean-up) settings in the cache plugin (every hour).

I only get the error message with the orange border when I try to submit the form with Internet Explorer and sometimes with Chrome. Firefox and Opera work well.

Here you can find the form:
http://c-schaefer-foto.de/fotoshooting-berlin/#buchen

Can you help me?

Best wishes,
Constantin

http://wordpress.org/plugins/contact-form-7/