Request::cookieValidationKey must be explicitly specified now.

config/web.php

@@ -8,7 +8,7 @@ $config = [
     'bootstrap' => ['log'],
     'components' => [
         'request' => [
-            // a secret key used to validate cookies. You may modify this key with your own one.
+            // !!! insert a secret key in the following (if it is empty) - this is required by cookie validation
             'cookieValidationKey' => '',
         ],
         'cache' => [