Unknown error occurs on every AJAX call in CP
This will be a environmental configuration issue. The only reason you could get that error is if your web server either isn't setting the global $_SERVER['HTTP_X_REQUESTED_WITH'] variable or it is setting it and it's a value other than XMLHttpRequest.

https://github.com/yiisoft/yii/blob/1.1.17/framework/web/CHttpRequest.php#L679

I'd check the server config and make sure there's nothing odd in your .htaccess files (if on Apache) that might be interfering.