Hello, I was getting this same error and have found a workaround (at least for dnn 4.3.2 using sqlexpress 2005 on the same server as IIS)
The problem for me was as follows:
In my web.config, I had the following:
<!-- Connection String for SQL Server 2005 Express -->
<add
name="SiteSqlServer"
connectionString="Data Source=MYMACHINE\SQLEXPRESS;Initial Catalog=dnn432;Integrated Security=True;Pooling=False"
providerName="System.Data.SqlClient" />
For some reason, it was unable to resolve the machine name, thus the following worked:
<!-- Connection String for SQL Server 2005 Express -->
<add
name="SiteSqlServer"
connectionString="Data Source=.\SQLEXPRESS;Initial Catalog=dnn432;Integrated Security=True;Pooling=False"
providerName="System.Data.SqlClient" />
Caveats: I am doing this for a local development machine, and am not saying this will work for all cases. It just was one case of this error and a workaround.
Hope that helps,
ee