Works for me now

The changes were made from my co-admin but he has no account here. I will send him the link for confirmation. As far as I can remember it was in the following lines:

// Purge table
		$wpdb->query( $wpdb->prepare( "
			DELETE FROM $wpdb->useronline
			WHERE (user_id > 0 AND user_id = %u)
            OR (user_id = 0 AND user_ip = %s)
			OR timestamp < DATE_SUB(CURRENT_TIMESTAMP, INTERVAL %d SECOND)
		", $user_id, $user_ip, self::$options->timeout ) );

Here is the reply of my co-admin:

The little change I made is the different handling of guests and logged in users. For registered users, you can distinguish the different user_id’s although if they are coming from the same ip.
WHERE (user_id > 0 AND user_id = %u)
The second part of the code is aimed to the guests (user_id = 0). Unfortunately, you can’t distinguish two or more guests coming from the same ip. Therefore I only can distinguish the different ip-addresses.
OR (user_id = 0 AND user_ip = %s)
the last part of the sql statement (concerning timeout) is kept.

You should remember, that I only made a quick hack. I did not check the code line by line and therefore it is possible that something else was damaged. But until now, everything works well.

Best reguards, the co-admin.