After I ran composer update to get new Yii2 framework version, I got this error

yii\web\Request::cookieValidationKey must be configured with a secret key

To overcome this error, I added this code below in config/main.php

'components' => [ 'request' => [ 'enableCookieValidation' => true, 'cookieValidationKey' => 'your-validation-key', ], ......

Hope it helps.