Merge forwardport of #11460 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/11460.patch (created by @diglin) based on commit(s):
  1. ac0406c
  2. 6ebf3ce

Fixed GitHub Issues in 2.3-develop branch:
  - #11140: Going to '/admin' while using storecodes in url and a different adminhtml url will throw exception (reported by @koenner01)

app/code/Magento/Store/App/Request/PathInfoProcessor.php

@@ -6,6 +6,7 @@
 namespace Magento\Store\App\Request;
 
 use Magento\Framework\Exception\NoSuchEntityException;
+use Magento\Store\Model\Store;
 
 class PathInfoProcessor implements \Magento\Framework\App\Request\PathInfoProcessorInterface
 {
@@ -42,7 +43,7 @@ public function process(\Magento\Framework\App\RequestInterface $request, $pathI
         }
 
         if ($store->isUseStoreInUrl()) {
-            if (!$request->isDirectAccessFrontendName($storeCode)) {
+            if (!$request->isDirectAccessFrontendName($storeCode) && $storeCode != Store::ADMIN_CODE ) {
                 $this->storeManager->setCurrentStore($store->getCode());
                 $pathInfo = '/' . (isset($pathParts[1]) ? $pathParts[1] : '');
                 return $pathInfo;