Service Desk 7.4 and newer
You are in your Application event log seeing the below error message:
It is not possible to run two different versions of ASP.NET in the same IIS process. Please use the IIS Administration Tool to reconfigure your server to run the application in a separate process
This means that you have two different ASP.Net versions set in the same IIS Process.
Go to IIS Management, highlight the Default Web Site and go to Advanced Setting.
Check to see which Application Pool has been selected on this.
Go to this Application Pool, highlight and select: Advanced Setting.
Make sure the ".NET Framework Version" is set to v4.0
After setting this, do an IISReset or Recycle the Application pool.