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:
$lang = $this->langcode_post_id( $the_post );
change that line to these:
$lang = ""; if ( function_exists( 'wpml_get_language_information' ) ) $lang = $this->langcode_post_id( $the_post );
That should get rid off the error message.Best,
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