Magento 2: Unable to unserialize value. Error: Syntax error

I’m creating Order export cron module for Magento 2.3. I’m getting below error

Cron-job "monthly_report" threw exception InvalidArgumentException    
In Json.php line 39                                              
Unable to unserialize value. Error: Syntax error

While debugging, I found below line produces error. I’m retrieving order details using OrderCollection Factory.

$order->getAllVisibleItems();

I tried below suggestions from stackexchange:

1. redis-cli flushall
2. bin/magento cache:clean