Invalid security or form key on admin page on magento 2

Go to admin page, login and got the error "Invalid security or form key. Please refresh the page."

Solution:

This is content of the patch file. Let's follow it

.../module-backend/App/Action/Plugin/Authentication.php  | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/vendor/magento/module-backend/App/Action/Plugin/Authentication.php 
b/vendor/magento/module-backend/App/Action/Plugin/Authentication.php
index 8227966e..0cdd9f12 100644
--- a/vendor/magento/module-backend/App/Action/Plugin/Authentication.php
+++ b/vendor/magento/module-backend/App/Action/Plugin/Authentication.php
@@ -102,6 +102,8 @@ class Authentication
 
 
    /**
+   * Ensures user is authenticated before accessing backend action controllers.
+   *
    * @param \Magento\Backend\App\AbstractAction $subject
    * @param \Closure $proceed
    * @param \Magento\Framework\App\RequestInterface $request
@@ -225,10 +227,9 @@ class Authentication
 
    // Checks, whether secret key is required for admin access or request 
    // uri is explicitly set
    if ($this->_url->useSecretKey()) {
-  $requestParts = explode('/', trim($request->getRequestUri(), '/'), 3);
-  $baseUrlPath = trim(parse_url($this->backendUrl->getBaseUrl(), PHP_URL_PATH), '/');
-  $routeIndex = empty($baseUrlPath) ? 0 : 1;
-  $requestUri = $this->_url->getUrl($requestParts[$routeIndex]);
+  // The requested URL has an invalid secret key and therefore redirecting 
+  // to this URL will cause a security vulnerability.
+       $requestUri = $this->_url->getUrl($this->_url->getStartupPageUrl());
    } elseif ($request) {
        $requestUri = $request->getRequestUri();
    }