HTTP Error When Uploading Images

Dear all, after updateing to the latest versions of ENFOLD (3.0.2) and WordPress (4.0–de_DE), I experience “HTTP-Error” when uploading an image. Besides, thousands of iterations of this image appear in the media library now. The image dimensions are 688px × 428px (png).

I already added this to wp-config.php, without any luck:
define('WP_MEMORY_LIMIT', '128M');

Any thoughts what else this might have caused?
I need to solve this problem, since the media library is unusable this way.
Where may I delete all this thousand images?

Kind regards