有没有很多细节在这里，但我得到了同样的错误，而试图启动调试一个Web应用程序项目在Visual Studio 2010中，我会3.5只变为4.0。

There aren't many details here, but I was getting that same error while trying to Start Debugging on a web application project in Visual Studio 2010 that I'd just changed from 3.5 to 4.0.

这解决了这个问题，我在命令行：

This solved the problem for me from the command line:

cd %windir%\Microsoft.NET\Framework\v4.0.30319
aspnet_regiis.exe -i

只要确保进入IIS管理器之后，并仔细检查你的应用程序池和这样的，因为很可能，这将重置你的一些配置。

Just make sure to go into IIS Manager afterwards and double-check your application pools and such, since it's likely that this will reset some of your configuration.