We have had the setup described above working for a few years now and as far as I can tell nothing major has changed in either the server setup or config.
We use ldap to connect to our on prem Active Directory, NTLM SSO setup through IIS and SSL turned on with a valid certificate.
This has all been working fine for years.
Now when user login the SSO kicks in and users get signed in but if they navigate anywhere else on the site a popup box appears asking for login credentials. The box appears 3 times and then goes away.
Any ideas would be gratefully received!