In IIS Manager, in the Advanced Settings for an Application Pool, there is an option called "Enable 32-Bit Applications". This setting is a lie, because setting it to True does not "enable 32-bit applications" but rather it "disables 64-bit applications".
Seriously? There is nobody on the IIS programming team who ever thought to change the name of that setting, so that millions of IIS users wouldn't be confused and waste hundreds of billions of dollars trying to figure out why their 64-bit application won't run? Seriously?
My suggestion is to change the setting name to "Enable applications compiled for:" and change the options to "32-bit" and "64-bit". And to be honest, this solution is so obvious, that once again I am not surprised that none of the thousands of computer "professionals" in Redmond could figure it out. They never do.
Microsoft is a failure; IIS is crap. Well, okay, IIS isn't as bad as, say, the actual Windows operating system, but it is still below the threshold of crappiness.