Connect to SQL Server from web application doesn't work?

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http://serverfault.com – I'm trying to connect to my SQL Server express instance from my ASP.NET web application but it doesn't work. The generic error message given to the application is: Cannot open database 'xxx' requested by the login. The login failed. Login failed for user 'xxx'. I can connect using those credentials from Management Studio Express and see the tables and data, but the connection through my web application just fails. I'm using the following connection string: Data Source=(local)\sqlexpress;User Id=xxx;Password=xxx;Initial Catalog=xxx; SQL Server Express is allowed to use remote connec (HowTos)