Have you set the https://docs.ckeditor.com/ckfinder/ckfinder3/#!/api/CKFinder.Config-cfg-connectorPath? Basic integration code should look like so:
`var editor = CKEDITOR.replace('editor1', {});
CKFinder.setupCKEditor(editor, { connectorPath: '/ckfinder/connector' });` and if the path changes on the server side, it should be updated on the client as well (although in your case, path will rather be default). Please also see point two of below link - https://docs.ckeditor.com/ckfinder/ckfinder3-net/quickstart.html#quickstart_installation_zip.
– j.swiderskiMay 29 '18 at 09:47