Hi Nagaraju
1. Have you changed any setting in the web dispatcher or portal system?
2. In Web dispatcher system have you enable the firewall? or have you installed any Antivirus software with Network firewall settings?
3. Kindly refer the SAP Notes
1911913 - 500 Dispatching error thru web dispatcher
1564699 - HTTP 500 Error when adding / removing rights in Access Levels
1928423 - “HTTP 500 Dispatching error” when a SAP instance calls a Web Dispatcher at the same server
Regards
Sriram