Magento 2.3 Single Page in Admin will not load

Question

I am trying to access Content -> Configuration -> Edit (my theme name) but as soon as i hit edit the page will load but get stuck on an Ajax loader gif. See screenshot.

I have cleared out the cache, cleared static content etc to no avail. Any idea?

Here's a copy of the browser console

JQMIGRATE: Migrate is installed with logging active, version 1.4.1
file-uploader.js:173 Uncaught ReferenceError: Base64 is not defined
    at UiClass.processFile (file-uploader.js:173)
    at Array.map (<anonymous>)
    at UiClass.setInitialValue (file-uploader.js:78)
    at UiClass.initialize (abstract.js:77)
    at UiClass.initialize (wrapper.js:109)
    at new UiClass (class.js:49)
    at Object.initComponent (layout.js:137)
    at fire (jquery.js:3232)
    at Object.fireWith [as resolveWith] (jquery.js:3362)
    at Object.deferred

(/maxcb/theme/design_config/edit/scope/stores/scope_id/1/key/ce4569e5b0258a5cf79d478b35922f4fd13fef7de215572ccdbf1bba235d09f6/anonymous function) (http://www.xxx.co.uk/pub/static/version1546980063/adminhtml/Magento/backend/en_GB/jquery.js:3461:37)

Answer 1

Taken from another site, this has fixed the problem - seems like it only effected version 2.3

Open those files

/vendor/magento/module-email/view/adminhtml/ui_component/design_config_form.xml /vendor/magento/module-catalog/view/adminhtml/ui_component/design_config_form.xml /vendor/magento/module-backend/view/adminhtml/ui_component/design_config_form.xml /vendor/magento/module-swatches/view/adminhtml/ui_component/design_config_form.xml /vendor/magento/module-theme/view/adminhtml/ui_component/design_config_form.xml

search for

formElement="fileUploader"

replace with

formElement="imageUploader" 

in all files

clear cache and reload.