After looking at the API code in JavaScript, it seems that this error is thrown if the channelUrl doesn't match the URL you're currently at.
To get round this, just set the channelUrl to a dynamic value that updates for every window
{
"channelUrl": window.location.protocol + "//" + window.location.host
}
If you run this in the console of your browser you'll see that it logs the base URL of the website you're currently on, and the error will no longer be thrown.
Update, this just delays the problem unfortunately. When it comes to authenticating the login it fails because of an unexpected domain on proxy url.
