Recently my search.cgi stopped working and don't know when it happended.

When I search a term in a browser, only a blank page is returned. If I reload the page, the result shows up. I tested in different browsers and got the same problem. I can search in shell without any problem.

If I turn on debugging, when I search, I got 500 error.

I don't know what is the problem. I even discard my search log, cache and reindex my tables, they makes no difference.

Any clues? Thanks.