I probably figured out the casue.
The plugin is compatible with WPML plugin, and in the current version I made a few changes related to that. The error message is referring to a function which tries to get the post language if WPML plugin is activated. Apparently some weird error happens when checking for WPML availability.
I have a possible fix, by adding one line of code to the plugin. Try opening the wp-content/plugins/ajax-search-pro/includes/etc./indextable.class.php file and go to line 657 (or close to it), where you should see this:
[code]$lang = $this->langcode_post_id( $the_post );[/code]
change that line to these:
[code]
$lang = "";
if ( function_exists( ‘wpml_get_language_information’ ) )
$lang = $this->langcode_post_id( $the_post );
[/code]
That should get rid off the error message.