What is error code 4820 and why response with this error code has content-type text/html?

What is error code 4820 and why response with this error code has content-type text/html?

Hello,

We noticed a new error code that ZohoCRM API returns some times: 4820. None of the documentation explains this code. The whole response is:
  1. {"response":{"error":{"message":"API call cannot be completed as you have exceeded the Rate limit","code":"4820"},"uri":"/crm/private/json/Contacts/downloadFile"}}
and its headers say that content type is "content-type"=>"text/html;charset=UTF-8", which makes it impossible to handle it correctly since it is clearly a JSON, not a html.

So, 
1. what this code means exactly?
2. how to make sure we don't hit it?
3. why its content type is declared as text/html?

Thanks!