asp.net mvc - Connection string between IIS and SQL server

Question

I'm confused by the connection string in the Web.config file of project with Entity Framework Web API. There are a lot of variants I tried, but none of them was helping me out. Currently the connection string is

  <connectionStrings>
       <add name="DefaultConnection" connectionString="Server=.SQLEXPRESS;Database=MyProject;User Id=John;Password=duck;" providerName="System.Data.SqlClient" />
  </connectionStrings>

I can access the database which is called MyProject in SQL Server Management Studio 2012 (even from remote), using John as username and duck as password.

The project is run by an IIS server on the same machine as the SQL server, the SQL server's instance name is SQLEXPRESS. My webpage is displayed on localhost, but the controller just replies 500 Interal server error whenever some data from the sql database is requested. It looks like there is no connection to the database.

What do I have to add or change in order to get a connection betweeen IIS and SQL server, or how can I locate better the problem's cause?

Answer 1

Use the Event Viewer to get more information on what the 500 error actually is.