HTTP Error 500.21 - Internal Server Error

HTTP Error 500.21 - Internal Server Error Handler "ScriptHandlerFactory" has a bad module "ManagedPipelineHandler" in its module list

If you have come across this error, its a good reason to be sad.
You have done a silly mistake.
Fortunately we have a working solution.
So show me some white marbles in uah mouth.

1. Check config file in your application, if its configured for AppPool to be INTEGRATED or CLASSIC
• Based on Config file, update Application Pool Managed Pipeline Mode to be either Classic or Integrated
2. It also happens when ASP.NET is not installed within IIS for a specific framework:
• Open command prompt as Administrator.
  
• Go to path

  c:\Windows\Microsoft.NET\Framework\v4.0.30319

• Run command

  aspnet_regiis -i

  
  

  c:\Windows\Microsoft.NET\Framework\v4.0.30319 aspnet_regiis -i
  Start installing ASP.NET (4.0.30319).
  .......
  Finished installing ASP.NET (4.0.30319).
  
  c:\Windows\Microsoft.NET\Framework\v4.0.30319
  

• Restart IIS and run the ASP.NET site