"The system cannot locate the resourc" on xmlhttp.send execution

[SushiSean]

When executing the xmlhttp.send function to request shipper for available
shipments (I send xml as string and receive result xml):

XMLHTTP xmlhttp = new XMLHTTP();
xmlhttp.open("POST", "https://gateway.fedex.com:443/GatewayDC",
false, "", "");
xmlhttp.setRequestHeader("Content-Type",
"application/x-www-form-urlencoded");

xmlhttp.send(strXML);

I got an error:

Server Error in '/' Application.
--------------------------------------------------------------------------------

The system cannot locate the resource specified.

Description: An unhandled exception occurred during the execution of the
current web request. Please review the stack trace for more information about
the error and where it originated in the code.

Exception Details: System.IO.FileNotFoundException: The system cannot locate
the resource specified.

Source Error:

Line 1586: "", "");
Line 1587: xmlhttp.setRequestHeader("Content-Type",
"application/x-www-form-urlencoded");
Line 1588: xmlhttp.send(strXML);
Line 1589: responsexml = xmlhttp.responseText;
Line 1590:

Source File: \\rc\release\Utilities\shipments.ascx.cs Line: 1588

I have load balancing running. The problem comes when load balancer switches
from Server1 to Server2. On Server 1 the same code executes successfuly.
Server2 is configured like Server1, and has no firewall.

I searched the forums, but did not find the exact answer. Can you suggest
the possible reasons of such error?