I just installed the DNN 5.6 on windows server 2008 with IIS7.0 / 
I'm able to login as admin but not as a superuser. Furthermore, when I look into the users db, the superuser account does not exist even though I created it during set up. Version 5.5 work fine on the same server. Any ideas? 

Yes, I did looked into the Users table, strange thing is that the host user account does not exist even though I created one during set up. The admin account was properly created but somehow not the host account. And yes, I did used  different logins for host and admin accounts. I also was able to replicate this error in 3 seperated intallations.   Admin account is entered in db but not host account, consequently can't login with host because as far as the DNN's db is concerned, host account does not exist.