Why does the validator show Valid JSON but json_last_error_msg shows syntax error?

Question

PHP

Sending long json js - php. On server

$calendar = json_decode($calendar,true, 512, JSON_PARTIAL_OUTPUT_ON_ERROR);
  if($calendar == null)
  {
    exit('error30 - bad json! -- '.json_last_error_msg());
  }

replies error30 - bad json! -- Syntax error
Checking json in online validator - Valid JSON
Can't find error, please help me.
JSON does not fit here - too long, maybe it's the length? (76689 characters)