I faced the same issue and solved by below steps:
Step 1 : Change the following file /vendor/magento/framework/Serialize/Serializer/Json.php especially the unserialize function.
Here is the original unserialize function -:
public function unserialize($string)
{
$result = json_decode($string, true);
if (json_last_error() !== JSON_ERROR_NONE) {
throw new \InvalidArgumentException('Unable to unserialize value.');
}
return $result;
}
Change the above function to the following -:
public function unserialize($string)
{
/* Added the following if clause to resolve the issue */
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 $result;
}
Also add the following is_serialized function in the same file /vendor/magento/framework/Serialize/Serializer/Json.php
function is_serialized($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;
}
Step 2: Use json_encode or better yet the method Magento\Framework\Serialize\Serializer\Json::serialize().
