Say what?
I got this error and was pretty much clueless. It is under Windows Server 2012 and it didn’t occur in 2008
HTTP Error 500.19 - Internal Server Error
The requested page cannot be accessed because the related configuration data for the page is invalid.
Detailed Error Information:
Module IIS Web Core
Notification BeginRequest
Handler Not yet determined
Error Code 0x80070021
Config Error This configuration section cannot be used at this path. This happens when the section is locked at a parent level. Locking is either by default (overrideModeDefault="Deny"), or set explicitly by a location tag with overrideMode="Deny" or the legacy allowOverride="false".
Config File \\?\C:\myapp\web.config
Requested URL https://localhost:443/api/v1/Login
Physical Path C:\myapp\api\v1\login
Logon Method Not yet determined
Logon User Not yet determined
Config Source:
90: < /modules >
91: < handlers >
92: < remove name="ExtensionlessUrlHandler-ISAPI-4.0_32bit" / >
More Information:
This error occurs when there is a problem reading the configuration file for the Web server or Web application. In some cases, the event logs may contain more information about what caused this error.
View more information »
How did I solve it?
Add Application Development from the Features! Remember to add the 4.5 stuff if you need it

And then do the following in a command prompt, if required. Update to Allow on the handlers and modules section in C:\Windows\System32\inetsrv\config\applicationHost.config and remember to open as administrator
< section name="handlers" overrideModeDefault="Allow" / >
< section name="modules" allowDefinition="MachineToApplication" overrideModeDefault="Allow" / >