Site upgrade from 1912 LTSR to 2507 LTSR fails with below exception 

 Error Source : Citrix Studio
    
    StackTrace: Citrix.Orchestration.Base.LogicModels.Exceptions.ScriptException Could not connect to the VDA Update service.
       at Citrix.Orchestration.Base.PowerShellSdk.HealthMonitor.Scripts.ReloadServiceInstancesScript.CheckForWorkingServiceSet()
       at Citrix.Orchestration.Base.PowerShellSdk.HealthMonitor.Scripts.ReloadServiceInstancesScript.RunScript()
       at Citrix.Orchestration.Base.PowerShellInteraction.PowerShellScript`1.Run()
       at Citrix.Orchestration.Base.PowerShellSdk.SiteService.Scripts.LoaderScript.InitializeSite(IProgressReporter loadServicesProgress)
       at Citrix.Orchestration.Base.PowerShellSdk.SiteService.Scripts.LoaderScript.RunScript()
       at Citrix.Orchestration.Base.PowerShellInteraction.PowerShellScript`1.Run()
       at Citrix.Console.DeliveryCenter.UI.ViewModel.DesktopStudioViewModel.LoadSiteState(IProgressReporter progressReporter)
       at Citrix.Console.DeliveryCenter.UI.ViewModel.DesktopStudioViewModel.ConnectToSite(String ccsLocation, Boolean safeForDispatcher, IProgressReporter progressReporter)
       at Citrix.Console.Common.UnsafeThreadHelper.InvokeSafeToBlock(Action action)
       at Citrix.Console.MmcSnapin.SnapInClass.OnLoadCustomSnapinData(AsyncStatus status, Byte[] persistenceData)
    

1. Close the Studio console.
2. On Delivery Controller open the PowerShell as administrator.
3. Run command "Set-VusDBConnection -DBConnection $null" to reset the service instance’s database connection string
4. Open the studio as administrator and start the site upgrade.
   
   
   
   

Site upgrade from 1912 LTSR to 2507 LTSR fails with exception "Could not connect to the VDA Update service."