I’ve been searching the web for a solution of my uploader in WordPress. It gives a http error when uploading png images, in the class-wp-image-editor-imagick.php file in the load function:
$this->image = new Imagick( $this->file );

WordPress version is  4.3.1
ImageMagick version 6.5.4-7
PHP Version 5.4.42

No idea if it is the combination of this can be the problem but I found a partial solution for the problem. In WordPress, go to Appearence -> Editor and open the functions.php file. Now enter this code snippet at the bottom:

add_filter( ‘wp_image_editors’, ‘change_graphic_lib’ );
function change_graphic_lib($array) {
    return array( ‘WP_Image_Editor_GD’, ‘WP_Image_Editor_Imagick’ );
}

This solution fixes the problem in /wp-admin/media-new.php but not on other pages like /wp-admin/post-new.php and /wp-admin/upload.php.

I’m investigating the problem with my hosting partner, will share a complete solution when available.

I’ve found the first part of my solution here:
https://wordpress.org/support/topic/http-error-when-uploading-images-but-not-all-failing