Open the file \vendor\magento\framework\Serialize\Serializer\json.php and replace the function unserialize() with the below

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 function as well

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;
 }