Use this util to validate you web.config connection string against the SQL server
Typical output:
Connecting using 'Data Source=(local);Initial Catalog=xxx;Integrated Security=True'
Successfully connected as 'DOMAIN\username'
Client name: MACHINENAME
Server version: 13.00.4202 ==> SQL Server 2016
Server Edition: Developer Edition (64-bit) (Enterprise)
Server collation: Danish_Norwegian_CI_AS
Successfully disconnected