MC-24840: Infinite redirect in case of backend URL is different from default website URL

Author: OlgaVasyltsun

app/code/Magento/Backend/App/Area/FrontNameResolver.php

@@ -120,10 +120,16 @@ public function getFrontName($checkHost = false)
      */
     public function isHostBackend()
     {
-        if ($this->config->getValue(self::XML_PATH_USE_CUSTOM_ADMIN_URL)) {
-            $backendUrl = $this->config->getValue(self::XML_PATH_CUSTOM_ADMIN_URL);
+        if ($this->scopeConfig->getValue(self::XML_PATH_USE_CUSTOM_ADMIN_URL, ScopeInterface::SCOPE_STORE)) {
+            $backendUrl = $this->scopeConfig->getValue(self::XML_PATH_CUSTOM_ADMIN_URL, ScopeInterface::SCOPE_STORE);
         } else {
             $backendUrl = $this->config->getValue(Store::XML_PATH_UNSECURE_BASE_URL);
+            if ($backendUrl === null) {
+                $backendUrl = $this->scopeConfig->getValue(
+                    Store::XML_PATH_UNSECURE_BASE_URL,
+                    ScopeInterface::SCOPE_STORE
+                );
+            }
         }
         $host = $this->request->getServer('HTTP_HOST', '');
         return stripos($this->getHostWithPort($backendUrl), (string) $host) !== false;