MSSQL Error 'The underlying provider failed on Open'

Question

I was using an .mdf for connecting to a database and entityClient. Now I want to change the connection string so that there will be no

Answer 1

Defining a new Windows Firewall rule for SQL Server (and for port 1433) on the server machine solves this error (if your servername, user login name or password is not wrong in your connection string...).