URGENT - ASP.NET, IIS, Windows Authentication troubleshooting required

This is an urgent requirement. I am looking for an experienced ASP.NET application developer to assist in troubleshooting authentication issues with ASP.NET 4.0 websites hosted on IIS 8.5 on a Windows Server 2012 machine.

We are using windows authentication on one website to generate a forms authentication ticket and users are then redirected to another website and should be logged in based on the ticket as long as they have a corresponding login in the users table (if not they are redirected to a login page)

This is an upgrade from the current production environment at our customer’s site (Server 2003, SQL 2008, IIS 6.5) which works fine to Server 2012, SQL 2012, IIS 8.5.

We have a fully working version in both our development and test environments however on deployment to our customers test environment we are experiencing issues where a valid user does not get authenticated and always ends up at the login form on redirect.

I believe we may have inconsistencies in configuration settings in IIS and web.config files and need assistance in troubleshooting and resolving these issues.