login loop

[JohanHWH]

We're moving a set of applications to new servers, building the servers from scratch.
On the webserver is a .NET application that connects to a separate server with ThinkTecture Identity Server 2.
After setting everything up, we experience a redirect loop after logging on to the web application.
As far as I can tell, the Identity server approves of the credentials, but then redirects to the base url of the web application and afterwards the loop is infinite.

We've looked at many settings, but are unable to locate the problem, meaning we need help tracking down what happens.

1. What information would be helpful to provide?
2. Where do I start further debugging?