Do you have by any chance another search plugin active, which can also override? Like Relevanssi, Searchwp or any other search filter or search plugin?
If so, then try deactivating them, because they all might have an override activated, and it can conflict. If still nothing changes, let me know and I will debug through the code.
I’ve deactivated Ajax Search Pro as well, but the same thing happens. It seems to be related to something else as well.
Also, can you please check the FTP permissions? I’ve tried to enable error reporting in your wp-config.php file to see what error messages it shows, but it does not let me save it. If you can’t change the permissions, at least can you please enable debug mode?