I'm not sure if this will be the right place to ask, however the DNN forums are not working for me at the moment and I have an unhappy client.
I set up DNN 5.2 on a Windows Server 2003 box. I installed SQL Server Express 2008 and then installed DNN. I went through the setup wizard and set up the user accounts, etc. I did a test login using the admin account in Firefox and everything checked out. The next day I get an email from the administrator over there saying he can't log in. Sure enough, when I use Internet Explorer (6, 7 or 8), the login attempt is marked as successful in the DNN event log, but the page still shows as if no one were logged in. Since it's IE-only, I'm really at a loss here and grasping at straws for answers. I see other people having the issue as well, but there's never a reason or solution posted.