Been seeing lots and lots of this error crop up recently in the osCommerce Forums. I also had a client with this exact same problem…
After a bit of investigation, the problem is that your /includes/configure.php file is calling resources from a SSL that does not exist.
Example; www.yoursite.com and https://yoursite.com – but you have not yet set up https on your domain.
Solution: stop using SSL connections until you have it set up!