I encountered this error when migrating a classic ASP application from a deprecated web server to its replacement.
I was getting an error complaining that an include file could not be found. After validating the path, and double checking the settings on the working server, the solution was obvious (though it took me a while to see it):
I had erroneously configured the virtual directory on the new server as a .NET application. As such, it was not properly handling includes.
To resolve this, go to the root path for the virtual directory in which your classic ASP site resides, and do the following:
1. Right-click on the folder, and select Properties
2. Next to the name of the Application (lower half of the 'Directory' tab), click Remove.
This should fix the problem. If not, there may be a deeper cause, but in my case, this did the trick.