Bad (or missing) ASP.NET registration map in IIS.
When multiple versions of the .NET Framework are executing side-by-side on a single computer, the ASP.NET ISAPI version mapped to an ASP.NET application determines which version of the common language runtime is used for the application. The ASP.NET IIS Registration tool (Aspnet_regiis.exe) allows an administrator or installation program to easily update the script maps for an ASP.NET application to point to the ASP.NET ISAPI version associated with the tool. The tool can also be used to display the status of all installed versions of ASP. NET, register the ASP.NET version coupled with the tool, create client-script directories, and perform other configuration operations.
(from systemroot\Microsoft.NET\Framework\versionNumber (for example: 4.xxxx))