How to fix the «Unable to unserialize value» failure while upgrading Magento from 2.1.x to 2.2.x?

[dmitrii_fediuk]

Replace the line

magento/magento2/blob/2.2.6/lib/internal/Magento/Framework/Serialize/Serializer/Json.php#L39-L39

with the following one:

$result = unserialize($string);