The ArcGIS Server Manager logon page displays "The page cannot be found" with HTTP 404 error. No other error messages display. REST displays the same message.
The Microsoft patch KB958481 changes the Web Service Extension for ASP.NET 2.050727 to 'Prohibited' in Internet Information Services Manager. ArcGIS Server Manager requires ASP.NET 2.05072 to be allowed.
Confirm this update is installed in Add or Remove Programs. Be sure 'Show Updates' is checked at the top. Scroll down to Microsoft .NET Framework 2.0 Service Pack 2. The KB958481 is listed there. The patch affects Windows 2003 and XP.