Connection loses on data-send. Server receives RST, ACK after handshake - sockets

Question

I've got a simple TCP-server hosted on 64bit windows server 2008 r2. TCP-server just receives connection and replies to incoming data with recieved message(echo). There are about 600-700 clients who try connect and send some information. And the problem is: server loses almost all of connections(about 90%) when data is sended from client to server(First 15-20 connections have been performed normallly). I've sniffed the TCP-traffic with Whireshark.
From server side log is:
+--------------+--------------+--------------------------------+
| Source | Destination | Info |
+--------------+--------------+--------------------------------+
| 1. client ip | server ip | [SYN] **Handshake step1** |
| 2. server ip | client ip | [SYN, ACK] **Handshake step2** |
| 3. client ip | server ip | [ACK] **Handshake step3** |
| 4. client ip | server ip | [RST, ACK] **Loses connection**|
+--------------+--------------+--------------------------------+
From client side log is:
+--------------+--------------+--------------------------------+
| Source | Destination | Info |
+--------------+--------------+--------------------------------+
| 1. client ip | server ip | [SYN] **Handshake step1** |
| 2. server ip | client ip | [SYN, ACK] **Handshake step2** |
| 3. client ip | server ip | [ACK] **Handshake step3** |
| 4. client ip | server ip | [PSH, ACK] Message |
| 5. client ip | server ip | [PSH, ACK] CRLF message |
| 6. server ip | client ip | [RST, ACK] **Loses connection**|
+--------------+--------------+--------------------------------+
In both cases the «Reset cause» is: \000\000\000......\000
The connection did not lose when we're connecting from local network.

Answer 1

I don't think it's related to your code, but I do have several questions:
1. What is the network speed between client and the server? Are there any packets lost for other applications? What's the size of the message sent from client?
2. How long is it between the RST received and the handshake finishes (server) or message sent (client)?
3. Do you know if there are any firewalls between the client and server? You also said it worked well on LAN. The China GFW often does so.

Answer 2

I found the solution. The problem was that, provider changed tariff plan without any notice. New tariff plan limited the maximum number of connections.