category number invalid errors

[hlcsky]

On some searches I get an error page saying
"Error: Category number is invalid. Incorrect settings file used?"

If i type random letters in the search form it seems to perform a search and find nothing.
But if I type a search which should yeild results I get the error.

Any ideas what is causing this? http://industrial-clay.com

[David]

I didn't see our software in use on the site referenced.

The most likely cause of the error is index corruption. Try re-indexing your site and uploading a new set of index files.