asp.net – 请求格式无法识别的URL意外结束在’/转换’发生后1-2

var xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
xmlhttp.open("POST","/xxx/Converter.asmx/Convert",false);
xmlhttp.setRequestHeader("Content-Type","application/x-www-form-urlencoded");
xmlhttp.send("conversionFolder=" + escape(conversionFolder));
if (xmlhttp.status == 200) {
  xmlDoc = new ActiveXObject("Microsoft.XMLDOM");
  xmlDoc.async = "false";
  xmlDoc.loadXML(xmlhttp.responseText);
  ... more stuff ...
  return str;
}
else {
  alert(xmlhttp.statusCode + " - " + xmlhttp.statusText);
}

当我记得在本地web.config中添加HttpPost协议时，所有工作正常：

<?xml version="1.0"?>
<configuration>
  <appSettings/>
  <connectionStrings/>
  <system.web>
    <webServices>
      <protocols>
        <add name="HttpPost"/>
      </protocols>
    </webServices>
    <compilation debug="false"></compilation>
  </system.web>
  <system.codedom>
  </system.codedom>
  <!--
    The system.webServer section is required for running ASP.NET AJAX under Internet
    Information Services 7.0.  It is not necessary for previous version of IIS.
  -->
  <system.webServer>
  </system.webServer>
</configuration>

但在一个生产服务器上运行1-2天后失败。它工作正常后asp.net过程被回收。它工作1-2天，然后它失败与这：

Exception information:
Exception type: InvalidOperationException
Exception message: Request format is unrecognized for URL unexpectedly ending in '/Convert'.

Request information:
Request URL: https://xxx/xxx/converter.asmx/Convert
Request path: /xxx/converter.asmx/Convert
User host address: 195.50.35.4
User: extranetkbk
Is authenticated: True
Authentication Type:
Thread account name: NT AUTHORITYNETWORK SERVICE

Thread information:
Thread ID: 14
Thread account name: NT AUTHORITYNETWORK SERVICE
Is impersonating: False
Stack trace: at System.Web.Services.Protocols.WebServiceHandlerFactory.CoreGetHandler(Type type,HttpContext context,HttpRequest request,HttpResponse response)
at System.Web.Services.Protocols.WebServiceHandlerFactory.GetHandler(HttpContext context,String verb,String url,String filePath)
at System.Web.Script.Services.ScriptHandlerFactory.GetHandler(HttpContext context,String requestType,String pathTranslated)
at System.Web.HttpApplication.MaterializeHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute()
at System.Web.HttpApplication.ExecuteStep(IExecutionStep step,Boolean& completedSynchronously)

为什么在失败前等待2天？我能做些什么来防止它失败？
有什么关系到这个服务器运行在HTTPS模式的事实吗？