PHP Warning: mysql_close(): 83 is not a valid MySQL-Link

Goodday,

After checking my log in open cart's admin i see the following line returning everyday and a lot of times per day.
PHP Warning: mysql_close(): 83 is not a valid MySQL-Link resource in /www/p/a/p/paperspecials.nl/public_html/system/database/mysql.php on line 76

I back traced the log and found out it started when i began using an extension from a collective shipping program.
This module is used to send shipping information to their site and make an shipping label and tracking number so i can ship the packages.
So i searched the forum for over an hour and even outside opencart forums, and a lot of modules are causing this warning but i don't seem to find or understand what the problem is causing exactly?
Can someone tell me what to look for in the supplied module to fix this warning?

Thank you very much.
Greetings Anton