I've copied my live DNN install to localhost for debugging, but it won't connect to my database. I'm finding it really hard to troubleshoot the issue because I'm not getting anything useful from the DNN -- just says it can't connect.

I've done all the usual.

Copied site to directory with network service modify access, added site to iis, modified hosts file. Site loads at dnndev.me, then hits the cannot connect error.

Integrated Security. Even tried creating sql login. No dice. SqlServer 2016.

I can access this database fine in SSMS with full permissions.

<add name="SiteSqlServer" connectionString="Data Source=.\SQLEXPRESS;Initial Catalog=rs1;Integrated Security=True" providerName="System.Data.SqlClient"/>

What am I missing? And even better, how do I troubleshoot this so I can figure out the issue in the future?