'Cheatin uh?' message in v3.8

I’ve recently set up a brand new site using WP3.8. I’ve been using the Simple Fields plugin to add meta boxes to page templates, and I created a few meta-boxes successfully. However, when trying to create some more, I get the ‘Cheating uh?’ message when I click save. And of course, all of the meta-boxes I just created have disappeared.

Please can someone advise me on how to get this sorted out, as I need to get this client site finished urgently.