Here are some items you can look at to fix the issue:
1. Look at handler mappings Is the cgi-exe present?
2. If not, you will have to install it.
3. If ISAPI-dll is not enabled, enable it and restart.
4. Then check the c:\inetpub\wwwroot\ILLiad folder for the illiad.dll file.
5. If it is not there, then you need to copy it over from the c:\illiad\setup\DLLUpdates\9.1 folder.
6. Restart IIS.
You should be working now.