I got the same error when recently upgrading, so i uninstalled the plugin completely and then re-uploaded and re-installed. I was getting the same error and googled for it, but found nothing that helped.
Eventually managed to edit the wp-useronline.php file and add // to comment out the function that was looking for the ip2nation convertion, the line has a comment preceding it now: $origin = get_user_origin($ip);
### Function: Output User’s Country Flag/Name
function ip2nation_country($ip, $display_countryname = 0) {
//$country_mirror = ‘http://lesterchan.net/wp-content/themes/lesterchan/images/flags’;
$country_mirror = plugins_url(‘wp-useronline/images/flags’);
$country_imgtype = ‘png’;
//$origin = get_user_origin($ip);
Seemed to work, although now without little nation flags, if you can live with that then that works.
