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".

Please see below for more information.

    <collapsible>true</collapsible>
                <label translate="true">HTML Head</label>
            </settings>
            <!-- Remove this code -->
            <!-- <field name="head_shortcut_icon" formElement="fileUploader">-->

            <!-- Add this code -->
            <field name="head_shortcut_icon" formElement="imageUploader">
                <settings>
                    <notice translate="true">Not all browsers support all these formats!</notice>
                    <label translate="true">Favicon Icon</label>

                <collapsible>true</collapsible>
                <label translate="true">Header</label>
            </settings>
            <!-- Remove this code -->
            <!-- <field name="header_logo_src" formElement="fileUploader"> -->

            <!-- Add this code -->
            <field name="header_logo_src" formElement="imageUploader">
                <settings>
                    <label translate="true">Logo Image</label>
                    <componentType>imageUploader</componentType>

Please check and let me know in case of any issue.

This is a Magento 2.3 bug, added wrong value for formElement for field header_logo_src

You can go to the file vendor/magento/module-theme/view/adminhtml/ui_component/design_config_form.xml

and in line number 154 you can see <field name="header_logo_src" formElement="fileUploader"> this you can change with <field name="header_logo_src" formElement="imageUploader">

But editing Magento core is not a good idea, for temporary fix till Magento release the patch you can use the following module I created just to rewrite design_config_form.xml

https://github.com/arshadpkm/magento-2.3-header-logo-upload-bug-fix

Assign permission to them,

Please Follow this link: https://github.com/magento/magento2/issues/10071