Please make sure you have correctly installed libxml on your server and that it is enabled as a PHP extension.
To check this, you can try this URL:
If it does not work, check your web service configuration (search shall not work either)
If it correctly returns XML in a web browser, then try to set format=html
If you get nothing then the trouble surely comes from XSLT transformation of autocomplete suggestions. This means that libxml is not correctly enabled.
Check that libxml is installed (below is an example from Linux Ubuntu):
Check your PHP configuration. You must have something like this:
If this still does not work, you may check that php_xsl.dll is enabled:
|In php directory, look for the ext folder, then look for php_xsl.dll, now open php.ini and put this at the bottom of the file: extension=php_xsl.dll|