Magento 2.2: Unable to unserialize value?

I was able to resolve this issue by Flushing my Redis Cache from the CLI

redis-cli flushall

I hope this helps future users.

The problem is in /vendor/magento/framework/Serialize/Serializer/Json.php there is a function unserialize($string) which gives you a syntax error if string is serialized (not json but php serialization).

There is a workaround - you can check if string is serialized (vs json-encoded) and then use serialize($string). Change unserialize to:

public function unserialize($string)
{
    if($this->is_serialized($string))
    {
        $string = $this->serialize($string);
    }
    $result = json_decode($string, true);
    if (json_last_error() !== JSON_ERROR_NONE) {
         throw new \InvalidArgumentException('Unable to unserialize value.');

    }
    return $result;
}

and add function to check if string is serialized:

function is_serialized($value, &$result = null)
{
    // Bit of a give away this one
    if (!is_string($value))
    {
        return false;
    }
    // Serialized false, return true. unserialize() returns false on an
    // invalid string or it could return false if the string is serialized
    // false, eliminate that possibility.
    if ($value === 'b:0;')
    {
        $result = false;
        return true;
    }
    $length = strlen($value);
    $end    = '';
    switch ($value[0])
    {
        case 's':
            if ($value[$length - 2] !== '"')
            {
                return false;
            }
        case 'b':
        case 'i':
        case 'd':
            // This looks odd but it is quicker than isset()ing
            $end .= ';';
        case 'a':
        case 'O':
            $end .= '}';
            if ($value[1] !== ':')
            {
                return false;
            }
            switch ($value[2])
            {
                case 0:
                case 1:
                case 2:
                case 3:
                case 4:
                case 5:
                case 6:
                case 7:
                case 8:
                case 9:
                    break;
                default:
                    return false;
            }
        case 'N':
            $end .= ';';
            if ($value[$length - 1] !== $end[0])
            {
                return false;
            }
            break;
        default:
            return false;
    }
    if (($result = @unserialize($value)) === false)
    {
        $result = null;
        return false;
    }
    return true;
}

After save fe. category without problem, You can restore class to default and there wont be such problem in future.

Do not edit core files for solution. Override following way Just put following line in di.xml inside etc directory

<preference for="Magento\Framework\Serialize\Serializer\Json" type="Namespace\ModuleName\Serialize\Serializer\Json" />

And inside Namespace\ModuleName\Serialize\Serializer Directory: file Json.php

<?php
namespace Namespace\ModuleName\Serialize\Serializer;



class Json extends \Magento\Framework\Serialize\Serializer\Json
{


    /**
     * {@inheritDoc}
     * @since 100.2.0
     */
    public function unserialize($string)
    {
      if($this->is_serialized($string))
        {
            $string = $this->serialize($string);
        }
        $result = json_decode($string, true);
        if (json_last_error() !== JSON_ERROR_NONE) {
             throw new \InvalidArgumentException('Unable to unserialize value.');

        }
        return $result;
    }


    function is_serialized($value, &$result = null)
    {
    // Bit of a give away this one
        if (!is_string($value))
        {
            return false;
        }
        // Serialized false, return true. unserialize() returns false on an
        // invalid string or it could return false if the string is serialized
        // false, eliminate that possibility.
        if ($value === 'b:0;')
        {
            $result = false;
            return true;
        }
        $length = strlen($value);
        $end    = '';
        switch ($value[0])
        {
            case 's':
                if ($value[$length - 2] !== '"')
                {
                    return false;
                }
            case 'b':
            case 'i':
            case 'd':
                // This looks odd but it is quicker than isset()ing
                $end .= ';';
            case 'a':
            case 'O':
                $end .= '}';
                if ($value[1] !== ':')
                {
                    return false;
                }
                switch ($value[2])
                {
                    case 0:
                    case 1:
                    case 2:
                    case 3:
                    case 4:
                    case 5:
                    case 6:
                    case 7:
                    case 8:
                    case 9:
                        break;
                    default:
                        return false;
                }
            case 'N':
                $end .= ';';
                if ($value[$length - 1] !== $end[0])
                {
                    return false;
                }
                break;
            default:
                return false;
        }
        if (($result = @unserialize($value)) === false)
        {
            $result = null;
            return false;
        }
        return true;
    }
}

Works perfectly