Server Error in '/RBSApp' Application. The request failed with HTTP status 503: Server Error

 How to fix this error?
 
The request failed with HTTP status 503: Server Error.
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.Net.WebException: The request failed with HTTP status 503: Server Error.

Source Error:

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace:

[WebException: The request failed with HTTP status 503: Server Error.]
   Microsoft.Reporting.WebForms.Internal.Soap.ReportingServices2005.Execution.RSExecutionConnection.GetSecureMethods() +179
   Microsoft.Reporting.WebForms.Internal.Soap.ReportingServices2005.Execution.RSExecutionConnection.IsSecureMethod(String methodname) +63
   Microsoft.Reporting.WebForms.Internal.Soap.ReportingServices2005.Execution.RSExecutionConnection.SetConnectionSSLForMethod(String methodname) +16
   Microsoft.Reporting.WebForms.Internal.Soap.ReportingServices2005.Execution.ProxyMethodInvocation.Execute(RSExecutionConnection connection, ProxyMethod`1 initialMethod, ProxyMethod`1 retryMethod) +754
   Microsoft.Reporting.WebForms.SoapReportExecutionService.LoadReport(String report, String historyId) +33
   Microsoft.Reporting.WebForms.ServerReport.EnsureExecutionSession() +107
   Microsoft.Reporting.WebForms.ServerReport.SetParameters(IEnumerable`1 parameters) +81
   Reports_ViewPDF.GenerateReport() in g:\SSCsourceDIR\RBSNew\Transactions\PFCommonControls\viewpdf.aspx.cs:74
   Reports_ViewPDF.Page_Load(Object sender, EventArgs e) in g:\SSCsourceDIR\RBSNew\Transactions\PFCommonControls\viewpdf.aspx.cs:24
   System.Web.UI.Control.OnLoad(EventArgs e) +106
   System.Web.UI.Control.LoadRecursive() +68
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +3785

Version Information: Microsoft .NET Framework Version:4.0.30319; ASP.NET Version:4.7.3770.0