If you got this error while trying to open the editor, please try to whitelist DragDropr IP address as it's explained here.
Also, please, check that the "Authorization" header is not being stripped from requests by your Magento server.
If the issue persists, you should enable the setting "Use Web Server Rewrites" under "Search Engine Optimization" group:
On the Admin sidebar, tap Stores. Then under Settings, choose Configuration.
In the panel on the left under General, choose Web and expand the Search Engine Optimization section and enable it.
Let us know if any additional questions arise!