[Mediawiki-l] AJAX and Error 500

[Jan]

Hello!

I wanted to try AJAX, so i switched it on in my MW 1.9.0:
$wgUseAjax = true;
$wgAjaxSearch = true;
$wgAjaxWatch = true;

Now, when I try to type the name of an article in the searchbox of my wiki, i 
get an error-message-popup:

Error: 500 Internal Server Error
Parser::replaceInternalLinks: $this->mTitle is null

Whats wrong?

Regards,
Jan