SQL Server Connection Problem
Your newer .NET Framework app seem to be using Windows authentication to connect to SQL Server
If that is your intent, make sure the app itself runs on user machines on behalf of domain user that has a Windows-authenticated login at SQL Server
For example, if your SQL Server has login [Domain/Name.Surname] account, make sure your newer .NET Framework app runs on behalf of that domain account, then it should be able to connect to SQL Server
p.s. I am seeing this error from time to time when our developers launch SSMS on behalf of their local Windows accounts and trying to use Windows authentication to connect to SQL Server, instead of launching SSMS on behalf of their domain user accounts (which have corresponding logins at SQL Server)
p.p.s. if your intent was actually use SQL authentication for connecting app to SQL Server, remove Integrated Security = True/SSPI from connection string...