I have the same problem. No mod security even installed on the server. Just to be sure I disabled it under Plesk by adding a vhost.conf file. No joy.
I am wondering if it might not be a permissions issue. The form displays fine and even shows different styles when you select them, but if you try and save anything it displays the mod security error and resets to default.
However the error logs are clean and not indicating any issues. Really weird.
Any help would be greatly appreciated because it looks like an awesome plugin.