API Error Code 20011 HeartBeat Configuration request failure.

Hello,

Our customer needs to use API for  PMS integration. 
When we send an API request to the Yeastar S300, it gives an error
"20011 HeartBeat Configuration request failure."

What is this means?
Our API request format is:

http://111.111.96.230:8088/api/v2.0.0/cdr/download?number=all&starttime=2019-10-24%2000:00:00&endtime=2019-10-24%2015:59:59&token=7919d2a3498dae73188d6d1979b29895&random=7919d2a3498dae73188d6d1979b29895

Please advice.
Thanks