Generating the SSRS report as pdf throws error

I have a report which renders a big report with graph and pie chart. When I try to export as pdf it throws 500 error.

From the SSRS logs I checked it has logged below error;

A client has disconnected from the Report Server Web service application domain ReportServer_SQL2016. No corrective action is required. An error code has been submitted to ASP.NET to release the connection. The error code is 800708CA. ---> Microsoft.ReportingServices.Diagnostics.Utilities.ReportServerHttpRuntimeClientDisconnectionException: A client has disconnected from the Report Server Web service application domain ReportServer_SQL2016. No corrective action is required. An error code has been submitted to ASP.NET to release the connection. The error code is 800708CA. ---> System.Runtime.InteropServices.COMException: This network connection does not exist. (Exception from HRESULT: 0x800708CA)

I have increased the http execution timeout in web.config, still same error. SQL and SSRS version is 2016