Jos,
Does this have to do with the Bootstrap RTE editor? If so, I think there might be something wrong with your Microflow. The image is not uploaded, but inserted in the text as a base64 encoded string. FYI: I just updated the widget fixing a couple of issues. The widget is submitted for approval, so I guess it will be updated in the AppStore today. Latest version can also be retrieved from Github
Found it. An BCo microflow in the entity stripped the HTML away and that resulted in this strange error.