This is being caused by a third party telemetry product.
See below for instructions
As per the Release Notes:
Unverifiable third party replacements of system DLLs (including D3D9.DLL and DINPUT8.DLL) utilized by the simulator will now result in loading errors. If you encounter a Loading Error #66, please remove any third party replacements of D3D9.DLL, DINPUT8.DLL or other imposter system DLLs from your iRacing installation. Do not delete this file from any other directory other than your iRacing folder.
Also, please try the following suggestions:
Log out of iRacing, close browser and open a new browser window.
Reboot your computer.
Run a Registry Cleaner on your machine. There are many free one's out there.
Update Windows to the latest service pack
Make sure you have the latest version of DirectX
Make sure your Cryptographic Service in Windows is enabled. You can find this in your Windows Control Panel...Administrative Tools...Services.