unserialize(): Error at offset 0 of 40 bytes Error

I got the same error, when I upgrade a Laravel 5.5 app to 5.6. The error comes form the EncryptCookies-Middleware.

Delete the cookies in your browser and/or clear your session-files in your Laravel app.

You have to set a news Key Generate because

php artisan key:generate

After that test again to run the Laravel Application

php artisan serve