Issue

Web application fails to connect to MS SQL Server 2017 Express with following error:

-2147217843 – [Microsoft][ODBC SQL Server Driver][SQL Server]Login failed for user ‘sql_user’.

Resolution

Issue was resolved by enabling mixed authentication in Microsoft SQL Server Management Studio:

• Right click on the server in Object Explorer > Properties > Security 
• Change Server authentication to SQL Server and windows Authentication mode.
  
• Restart SQL instance. 

Windows Server 2016
Microsoft SQL 2017 Express Edition