Okay, I am at a loss on this issue. I have a client who has three servers. A 2012R2 DC, A 2008R2 SQL Server, and a 2012R2 Terminal Server. The SQL Server is running SQL 2008
The VAR software vendor that runs a SQL app on the server server did an upgrade to their software. During the upgrade process someone evidently logged into the SQL server app. Everything on the network, domain and SQL server and this app was working fine with no issues before this upgrade. After the upgrade many things with their app and SQL stopped working. They blame it on the users logging into the SQL app during the upgrade and are saying it is a domain, DNS or AD issue even though everything worked before their upgrade and nothing was changed at all with the domain, AD or DNS. The main issue is you can no longer log into their SQL app using Win NT Authentication from anywhere other than on the SQL server itself. You can use their app to login to the SQL server on the physical SQL server using Win NT Auth just fine. If you try to do it from any pc or the other two servers you get "could not establish a connection to the database" error. You can use their SQL app to login from anywhere if you use SQL auth and the sa credentials.
You can log into the SQL server with domain credentials just fine, just as you can any pc or the other servers. Users have no issues logging into the domain with their domain credentials, locally or even remotely. You can add new users, new computers, etc to the domain just fine. You can ping the SQL server with ip and name no problems from the other servers and all pc's. DNS and nslookup commands on the SQL server work from the other servers and all pc's. SQL server has named pipes and IP enabled. Remote connections is enabled. The ip's are correct. Using the port query tool and running query on SQL service of the SQL server from either of the other servers all pass.
Looking at permissions on the database the domain users account has permissions to the db.
The SQL server service is using the Local System account for login. There is only one instance - the default
I have another client using this same setup, same VAR software and version who got the upgrade but they work fine. I have tried to compare all settings in the domain and SQL between the two to find a difference but have not yet.
Any suggestions on more things I can look at or try? Anything I can run to do more tests? I am not a SQL expert by any means and have looked at everything I know. Thanks.