Error in call to API function "files/list_folder": request body: could not decode input as JSON

Good morning,
I do this request with Arduino:

  client.print(String("POST /2/files/list_folder HTTP/1.1\r\n") +
               "Host: api.dropboxapi.com" +
               "User-Agent: ESP8266/Arduino_YAGI\r\n" +
               "Authorization: Bearer " + (String)my_token +"\r\n" +
               "Accept: */*"+
               "Content-Type: application/json" +
               "Content-Length: 33" + "\r\n" + "\r\n" +
               "Dropbox-API-Arg: {\"path\":\"/glucose/201901/09/15/\"}"
               ); 

so, the arg is:

               Dropbox-API-Arg: {"path":"/glucose/201901/09/15/"}

The 'client' variable is:

               WiFiClientSecure client;

where 'WiFiClientSecure' is a package of the library to do secure calls.

But I receive this error:

HTTP/1.1 400 Bad Request
Server: nginx
Date: Thu, 10 Jan 2019 09:53:23 GMT
Content-Type: text/plain; charset=utf-8
Transfer-Encoding: chunked
Connection: keep-alive
Content-Security-Policy: sandbox; frame-ancestors 'none'
X-Dropbox-Request-Id: xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
X-Frame-Options: DENY
X-Content-Type-Options: nosniff
Content-Disposition: attachment; filename='error'

5f
Error in call to API function "files/list_folder": request body: could not decode input as JSON

Any idea to resolve this error? 

Thank you very much in advance!