After installation, "Manager" webpage pops up (or you can do Desktop Start > All programs > ArcGIS > ArcGIS 10.1 for Server > Manager). web address has "localhost", so I overwrited with my server name. It didn't work and I got same error mesage.
After I read rkilinski's #6 thread, I checked the log file. My log file said my machine name has underscore that I named originally. Also I noticed a folder inside "logs" folder had original server name not current server name.
ESRI suppoort asked me to uninstall ArcGIS Server and re-install it. So after uninstalled the software, I checked the server name folder (it was still there) under "logs" folder, and I renamed to current server name.
I re-installed ArcGIS Server and tried to make new site but I got same error message again. I checked the server name folder under "logs" folder. New folder with original server name (not current machine name) was created. So I was curious about registry. In the registry, I found original server names in several locations. I am suspicious that maybe one of registry is causing the problem. I am waiting ESRI support reply. I don't want touch registry without knowing exact cause.