I have same error with nancy.
When click submit button, the form was alert my html code.
‘<!DOCTYPE html>
<html lang=”en-US”>
<head>
<meta http-equiv=”Content-Type” content=”text/html; charset=UTF-8″ />
<meta name=”viewport” content=”initial-scale=1.0,width=device-width” />
……..
</body>
</html>
‘
actually this.responseText was ‘myhtmlcode’: line 111
wp version: 3.9.2
plugin version: 1.8.2