CKEditor - cross-domain file upload with KCFinder, what could be wrong?

Question

JavaScript

There is a domain on which the CKEditor script is located: engine.domain.com
There are subdomains that use this script: like subdomain.domain.com

The KCFinder plugin is attached to CKEditor (the first two letters are not confused) for uploading files.

There are no problems with loading. Problem with inserting file into text. Double clicking on a file in the manager raises the following error: Uncaught TypeError: Cannot read property 'tools' of

undefined

if (_.opener.name == "ckeditor") {
        _.opener.CKEDITOR.tools.callFunction(_.opener.CKEditor.funcNum, fileURL, "");
        window.close();
}

What could be the problem? Who faced, maybe?