Hallo,
nice to be back to the community after a long time!
I'm facing a very singular issue. I'm copying a production install of a DNN 7.3.3 to my local development machine.
I have SQL Server 2012 installed in localhost with IIS on my local machine, while I have 2 separate servers in production.
I have other 2-3 DNN installs on my local machines which regularly access the database, that I use for module development.
I need to do the same with this new site.
I modified the web.config connection strings to point at "localhost" for the DB server(which is already working fine for the other DNN installs so no TCP/IP or firewall issues).
I restored the DB inside SQL Server, recreated the user login and fixed the orphaned users to re-associate the login to the DB user.
Things I do normally, so nothing new here really.
About folder security I associated the root folder to the IIS application pool tied to the local website I created (which by the way is the same application pool of the other websites that actually do work fine).

So I'm really lost at this point.
Looking at the logs I just have:
2017-01-29 18:15:56,049 [MyPC-PC][Thread:22][ERROR] DotNetNuke.Common.Initialize - The connection to the database has failed, however, the application is already completely installed, a 500 error page will be shown to visitors. No other clue.

I restarted the IIS, I restarted SQL Server, I restarted the whole machine. No luck.

Some hint before I really go mad would be really appreciated.
Thanks in advance

Yes my friend, changed both to localhost as for the db server location.

I don't have SQL Express installed.

Those are both the SQL strings of the NON WORKING site:

<add name="SiteSqlServer" connectionString="Data Source=localhost;Initial Catalog=mycatalog.com;User ID=my.user.com;Password=123abc" providerName="System.Data.SqlClient" />

<add key="SiteSqlServer" value="Data Source=localhost;Initial Catalog=mycatalog.com;User ID=my.user.com;Password=123abc" />

And this is (one of the two) SQL strings of the working site on the same local machine:

<add name="SiteSqlServer" connectionString="Data Source=localhost;Initial Catalog=dnndev.me;User ID=dnndev.me;Password=dnndev.me" providerName="System.Data.SqlClient" />