Submit button not working

We LOVE this plugin, and I use it on several sites I manage. However, the submit button on our contact form is not working on this site.
http://www.milewisctr.org/get-updates/

I have tried disabling plugins one by one but apparently none of the plugins are causing the issue.

Thanks in advance for your help.

http://ww.wp.xz.cn/plugins/si-contact-form/