You will get a 401.1 error when trying to access the fully qualified domain name (FQDN) of your SharePoint site from the server. This is due to the loopback security check found in Windows Server 2003 SP1 or later, as well as Windows Server 2008 and Windows Server 2008 R2. To fix this you need to add a registry key to disable the loopback check. You could do this via the Registry Editor, but where is the fun in that?! Pop open a PowerShell console and enter the following:
New-ItemProperty HKLM:\System\CurrentControlSet\Control\Lsa -Name "DisableLoopbackCheck" -Value "1" -PropertyType dword
Make sure to reboot your server afterwards. Then, try your SharePoint site again and you should be good to go!
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SharePoint Adam 
05/06/2010 at 10:21 pm
very neat solution!
06/23/2010 at 6:49 am
Watch out – one of the double quotes was transformed to a question mark for me on a copy/paste
06/23/2010 at 7:42 am
Thanks for catching that. I have updated the post and believe it will copy/paste fine now.
08/20/2010 at 12:33 am
Thanks a lot, this really helped!!!
01/17/2012 at 11:30 am
Thanks this is going to my installation script rigth now
04/02/2012 at 2:18 am
Great ! very handy….
02/12/2014 at 1:56 pm
Hey! how about a shutdown -r -t 0 to restart the server in just one script block?