Let's say you created a web application with a host header on SharePoint which is installed on Windows Server 2008, and your credentials can not be authenticated if you try to log in to the web application on the server itself. Check the “Security” category on the event logs, if you see some entry like the one below, then go on reading 
An account failed to log on.
Subject:
Security ID: NULL SID
Account Name: –
Account Domain: –
Logon ID: 0×0
Logon Type: 3
Account For Which Logon Failed:
Security ID: NULL SID
Account Name: username
Account Domain: domainname
Failure Information:
Failure Reason: An Error occured during Logon.
Status: 0xc000006d
Sub Status: 0×0
Process Information:
Caller Process ID: 0×0
Caller Process Name: –
Network Information:
Workstation Name: clientname
Source Network Address: clientip
Source Port: portnumber
Detailed Authentication Information:
Logon Process:
Authentication Package: NTLM
Transited Services: –
Package Name (NTLM only): –
Key Length: 0
This problem has occured with the recent patches ( I do not know which one caused this ). The main reason for this issue is, the system blocks the authentication procedure while resolving the host header given to the web application. To resolve this issue, a modification must be done to the server's registry. You can read the article here.
kurtlubakla