Recently encountered an issue in SQL when running SQL Agent Jobs would give the following error in the agent jobs log:
Could not obtain information about Windows NT group/user ‘domainuser’, error code 0x5. [SQLSTATE 42000] (Error 15404)(ConnIsLoginSysAdmin)
The solution to this is to add the SQL Server service account to the ‘BUILTINWindows Authorization Access Group’ on your DC.
You can then run “EXECUTE AS USER = ‘domainuser’” on SQL Server to check.