Data Source=.; needs to be filled with your SQL server name or IP address.

e.g.

<add name="SiteSqlServer" connectionString="Data Source=MyServer;Database=prueba-dnn;uid=prueba-dnn;pwd=pepe.griyo" providerName="System.Data.SqlClient" />

Please note that if that's the username & password to your database, you should change it since you posted it to a public forum.

I found the problem! The connection string was not wrong, the problem was that the ASP.NET application was running under .NET framework version 1.1, and the web.config file (and framework infrastructure) changed in 2.0. The framework that was trying to run the application (1.1) did not know aboyut the changes in the configuration sections in the new version, so it prompted the syntax errors.

In the end, what i did to solve the problem, was to register again the script maps in all ASP.NET applications under the path of DDN like this:

aspnet_regiis.exe -s W3SVC/1/ROOT/DNN (in my case..)

And now it reads correctly the configuration file.