What (web) server are you moving the site from?

We are moving from

32-bit Windows Server  2003 R2 with IIS6

to

64-bit Windows Server 2008 R2 with IIS7

Then you are possibly facing incompatibilities with web.config. The IIS6 and IIS7 web.config use different elements for many configuration tasks. My suggestion is you do a clean install to get a web.config file from the new server. Then copy your application to the new server and start, using the web.config from the installation as a template, to build your IIS7 compatible web.config and verify that it works.

Edit: To clarify, you can't just diff the web.configs and copy differences that your application has. Things you have configured in the IIS6 web.config you might have to do in a different way in IIS7 to acheive the same result. If you use addon products and frameworks I suggest you install those as well in the demo site to get the IIS7 config for those products aswell.

Yup, I agree. Get a demo site up first to make sure you got all settings correct in web.config / file access rights etc (IUSR and IIS_USR etc). When that works, set up your own site in the same spot and use that web.config as a template for you new web.config