A connection string is configured but Umbraco could not connect to the database

Questions
1

I am trying to connect to a local database using a simple Umbraco v8 project but am unable to due to the error:

Boot failed: Umbraco cannot run. See Umbraco’s log file for more details.

-> Umbraco.Core.Exceptions.BootFailedException: A connection string is configured but Umbraco could not connect to the database.
  at Umbraco.Core.RuntimeState.DetermineRuntimeLevel(IUmbracoDatabaseFactory databaseFactory, ILogger logger) in d:\a\1\s\src\Umbraco.Core\RuntimeState.cs:line 194
  at Umbraco.Core.Runtime.CoreRuntime.DetermineRuntimeLevel(IUmbracoDatabaseFactory databaseFactory, IProfilingLogger profilingLogger) in d:\a\1\s\src\Umbraco.Core\Runtime\CoreRuntime.cs:line 290
  at Umbraco.Core.Runtime.CoreRuntime.Boot(IRegister register, DisposableTimer timer) in d:\a\1\s\src\Umbraco.Core\Runtime\CoreRuntime.cs:line 169

I have used a fair few connection strings in the Web.config file: the most recent being:

<add name="umbracoDbDSN" connectionString="Server=xxxxxxx; Database=Docs; user id=Docs; password=1234567890" providerName="System.Data.SqlClient" />

Is there anything wrong with the syntax above - I have tried about 50 other combinations of Connection string and none have worked.

and yes… Windows and SQL Authentication mode box has been ticked.

Thanks

This is a companion discussion topic for the original entry at https://our.umbraco.com/forum/113653-a-connection-string-is-configured-but-umbraco-could-not-connect-to-the-database