This problem is encountered when you edit and save UltimateSearch.config in VS.NET due to .NET caching. You can verify this issue by checking the UltimateSearch.event.log file to see "The process cannot access the file "C:\inetpub\wwwroot\YourWebApp\UltimateSearchInclude\UltimateSearch.config" because it is being used by another process".
You should always edit UltimateSearch.config in Notepad instead of VS.NET since VS.NET puts a lock on the file that prevents it from even being read. However, if you edit and save the config file in VS.NET, and experience any problems (check UltimateSearch.event.log in case of an unwanted behavior) follow these steps in the given order:
1. Rebuild (not build) your application.
2. Delete the UltimateSearch.index file.
3. Run your application and search.
Note that you don't need to restart IIS, or reboot your machine.