SQL Server Connection Problem I'm completely stuck on a connection problem that we're experiencing for users connecting to our SQL Server instance from a particular domain using a particular application. The error that we're seeing is (from Windows Event log):
2020-11-30 00:11:22.33 Logon       Error: 17806, Severity: 20, State: 14.
2020-11-30 00:11:22.33 Logon       SSPI handshake failed with error code 0x8009030c, state 14 while establishing a connection with integrated security; the connection has been closed. Reason: AcceptSecurityContext failed. The Windows error code indicates the cause of failure. The logon attempt failed   [CLIENT: xxx.xxx.xxx.xxx]
2020-11-30 00:11:22.33 Logon       Error: 18452, Severity: 14, State: 1.
2020-11-30 00:11:22.33 Logon       Login failed. The login is from an untrusted domain and cannot be used with Windows authentication. [CLIENT: xxx.xxx.xxx.xxx]

Users are able to connect to the database over the same session from an old legacy app that uses the old ADO database driver. But our newer .NET Framework app is unable to connect due to this error. Also, it works when I run the app from my sysadmin account. As a hapless developer with a limited understanding of SQL Server security I'm a bit out on thin ice when it comes to further diagnosing the problem here. Does anyone have any ideas what could be causing this 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...