CRITICAL Error: Unable to serialize value

When I upgraded the Magento on the checkout page and that was “report.CRITICAL: Unable to serialize value. Error: Malformed UTF-8 characters, possible incorrectly encoded.”

Please follow below step:

1)  Create registration.php file at Path :- app/code/Dolphin/Serializer

<?php
\Magento\Framework\Component\ComponentRegistrar::register(
    \Magento\Framework\Component\ComponentRegistrar::MODULE,
    'Dolphin_Serializer',
    __DIR__
);

2) Create module.xml file at  Path :- app/code/Dolphin/Serializer/etc

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:Module/etc/module.xsd">
    <module name="Dolphin_Serializer" setup_version="1.0.0">
        <sequence>
            <module name="Magento_Framework"/>
        </sequence>
    </module>
</config>

3) Create di.xml file at path :- app/code/Dolphin/Serializer/etc

<?xml version="1.0" ?>
<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Framework\Serialize\Serializer\Json" type="Dolphin\Serializer\Rewrite\Magento\Framework\Serialize\Serializer\Json"/>
    <preference for="Magento\Framework\Serialize\Serializer\JsonHexTag" type="Dolphin\Serializer\Rewrite\Magento\Framework\Serialize\Serializer\JsonHexTag"/>
</config>

4) Create Json.php file at path :- app/code/Dolphin/Serializer/Rewrite/Magento/Framework/Serialize/Serializer

<?php

namespace Dolphin\Serializer\Rewrite\Magento\Framework\Serialize\Serializer;

class Json extends \Magento\Framework\Serialize\Serializer\Json
{
    public function utf8ize($mixed)
    {
        if (is_array($mixed)) {
            foreach ($mixed as $key => $value) {
                $mixed[$key] = $this->utf8ize($value);
            }
        } elseif (is_string($mixed)) {
            return mb_convert_encoding($mixed, "UTF-8", "UTF-8");
        }

        return $mixed;
    }
    /**
     * @inheritDoc
     * @since 101.0.0
     */
    public function serialize($data)
    {
        $result = json_encode($this->utf8ize($data));
        if (false === $result) {
            throw new \InvalidArgumentException("Unable to serialize value. Error: " . json_last_error_msg());
        }
        return $result;
    }

    /**
     * @inheritDoc
     * @since 101.0.0
     */
    public function unserialize($string)
    {
        $result = json_decode($string, true);
        if (json_last_error() !== JSON_ERROR_NONE) {
            throw new \InvalidArgumentException("Unable to unserialize value. Error: " . json_last_error_msg());
        }
        return $result;
    }
}

5) Create JsonHexTag.php file at path :- app/code/Dolphin/Serializer/Rewrite/Magento/Framework/Serialize/Serializer

<?php

namespace Dolphin\Serializer\Rewrite\Magento\Framework\Serialize\Serializer;

class JsonHexTag extends \Magento\Framework\Serialize\Serializer\JsonHexTag
{
    /**
     * @inheritDoc
     * @since 102.0.1
     */
    public function serialize($data): string
    {
        $data = $this->utf8convert($data);
        $result = json_encode($data, JSON_HEX_TAG);
        if (false === $result) {
            throw new \InvalidArgumentException('Unable to serialize value.');
        }
        return $result;
    }

    public function utf8convert($mixed, $key = null)
    {
        if (is_array($mixed)) {
            foreach ($mixed as $key => $value) {
                $mixed[$key] = $this->utf8convert($value, $key);
            }
        } elseif (is_string($mixed)) {
            $fixed = mb_convert_encoding($mixed, "UTF-8", "UTF-8");
            return $fixed;
        }
        return $mixed;
    }
}

Hope this helps. Thanks.