This error occurs if the application pool running the VETtrak API in IIS is set to .NET Framework version 4. It needs to be changed to use .NET Framework version 2. If version 2 is not available, .NET Framework 3.5 needs to be installed.
Troubleshooting: "There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler' section defined" error when trying to open the asmx in a web page
Ash Wayling