Can't save list settings – 404 error

Hi

Successfully linked to my Constant Contact account but when I choose a list for the form to post to and click Save Settings – I get a 404 error – page does not exist – and the settings are not saved.

Might this be a cache issue? (WP Super Cache installed)

Thanks
Mark

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