ENGCOM-4397: Infinite redirects in Magento admin #21454 #21455

 - Merge Pull Request #21455 from Jitheesh/magento2:feature/admin-redirect-21454
 - Merged commits:
   1. 11cfe04

Author: magento-engcom-team

app/code/Magento/Backend/App/Request/BackendValidator.php

@@ -146,8 +146,9 @@ private function createException(
                 $exception = new InvalidRequestException($response);
             } else {
                 //For regular requests.
+                $startPageUrl = $this->backendUrl->getStartupPageUrl();
                 $response = $this->redirectFactory->create()
-                    ->setUrl($this->backendUrl->getStartupPageUrl());
+                    ->setUrl($this->backendUrl->getUrl($startPageUrl));
                 $exception = new InvalidRequestException(
                     $response,
                     [