Sending data via POST does not have a limit except for the one you have set in your server (php_value post_max_size 400M);
you might want to check your timeout value, try increasing your current value:
curl_setopt($ch, CURLOPT_CONNECTTIMEOUT, 60);
curl_setopt($ch, CURLOPT_TIMEOUT, 60);
This might be closing your connection in server A before the entire request is sent to the server B.
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