Software Version: Metasys UI 12.0
Error Code: HTTP Error 500.21 - Internal Server Error
Solution 1: Open PowerShell as Administrator and run Install-WindowsFeature Web-Asp-Net45.
Solution 2: In IIS Manager, select the UI or UIOffline application under Default Web Site. Go to Basic Settings and ensure Application pool is UI_AppPool. Select Application Pools, choose UI_AppPool or UIOffline_AppPool, go to Basic Settings, and change Managed pipeline mode from Classic to Integrated.