AJAX error when uploading image-file

Problem/Motivation

When using the default field image widget and try to upload an image, an AJAX-error is thrown.

Uncaught Drupal.AjaxError {message: "↵An AJAX HTTP request terminated abnormally.↵Debug…=1↵StatusText: error↵ResponseText: ↵ReadyState: 0", name: "AjaxError"}
POST https://stm6040865688fe1-bnbpjepv2lkqkjagjotslls6z4eqbbu1.tugboat.qa/node/add/article?element_parents=field_image/widget/0&ajax_form=1&_wrapper_format=drupal_ajax net::ERR_FAILED

Normally the image should be uploaded and a preview-thumbnail should be visible.

Steps to reproduce

Just install a fresh D8 core and try to upload an image to the default article content type. => /node/add/article
Check the Console in Inspector.