That plugin has wrong theme hook data
'selector' => 'html > body',
'type' => 'add_before',
Simple output is
<!DOCTYPE html>
<html lang="en-US" dir="ltr">
<head>
<meta http-equiv="Content-Type" content="text/html;charset=UTF-8">
<title>Forums - IPS Community Suite</title>
</head>
<p>plugin code</p>
<body>
</body>
</html>
It's not valid
Change
'type' => 'add_before',
to
'type' => 'add_inside_start',