You’re logged directly into a SQL Server 2012 server with an account that belongs to the domain admins group, launch SQL Server Management Studio and attempt to use Windows Authentication to connect to a database instance but notice that you receive the following error:
Cannot connect to <SQLserverName>\<DatabaseInstance>.
Login failed for user ‘<domainName>\<userName>’. (Microsoft SQL Server, Error: 18456)
You’ve confirmed that the Domain Admins group is assigned to the Security > Logins folder and assigned sysadmin permissions:
You noticed that you do not have this problem if you directly add the account you’re logged in as to the logins folder.
One of the reasons why this issue occurs is if the SQL server exhibiting this behavior has UAC turned on. If this is the case, the problem will go away if SQL Server Management Studio as ran as an administrator.