Windows Authentication No Longer Working

File Vista was functioning properly on Windows 2003 server in a domain.  Authentication was handled through IIS using domain authentication in the following format:
NETBIOSDomainName\username

We had to rebuild the server.  In doing so, we installed Windows 2003 R2 Standard.

I installed File Vista 2.6 and authentication is no longer functioning.  Any suggestions as to where I can begin looking?

We have Unlimited Users License installed.

Update

No error is given.  When a user logs on as  DMZ\joe, they are redirected to the File Vista login screen where they must re login as DMZ\joe using the File Vista password, even though they are already authenticated by IIS.

Please open web.config and change the line

<authentication mode="None" />

to 

<authentication mode="Windows" />

Since version 2.5, authentication mode is set to "None" by default. This is to assure that FileVista's own authentication is used by default especially on IIS7. Since you need to use domain authentication and bypass FileVista's authentication, you need to set the mode to "Windows".