Why did I get the error "Unable to load the binding classes" with the .NET framework?

PayPal can't load binding classes with the .NET framework.

When instantiating the proxy classes, the .NET Framework creates an XML serialization assembly. This operation requires READ and WRITE access to the directory specified in the TEMP environment variable for the user account running your web application.

To access these permissions, right-click on the TEMP folder and select Properties > Security. Make sure that the Read and Write properties have the Allow box checked for the appropriate user account.