Magento 2.3.0 “A technical problem with the server created an error.” When uploading the logo image

 This is the bug in the Magento 2.3 version, but you can fix the issue by the change in the below code.

You can go to this file: /vendor/magento/module-theme/view/adminhtml/ui_component/design_config_form.xml and then remove "fileUploader" and replace it with "imageUploader".