Hi Steve,
Thank you for letting me know. I was able to re-create the issue on certain conditions.
To fix it, simply replace the wp-content\plugins\ajax-search-pro\includes\classes\ajax\class-asp-autocomplete.php file contents on your server with this via FTP.
Before editing, please make sure to have a full site back-up just in case!
Let me know if you need help with it!