Figured it out! On line 30 of imagecrop.php:
<form class="ngg_crop_form" action="<?=$_SERVER['REDIRECT_URL']?>" method="post">
I changed it to:
<form class="ngg_crop_form" action="<?=$_SERVER['REQUEST_URI']?>" method="post">
And the plugin works. Eventually some servers don’t support $_SERVER[‘REDIRECT_URL’]. I’m on Hostgator and I guess they don’t.