Have you change something that could affect this button to throw an error? Is your https part of the store setup right? I just did a test order on my site and cannot reproduce the error. The button sends to
https://home without errors. If you insist you can change the code for the button to redirect to "$http_location/home.php" instead of home.php but I think that your customers should stay in https as long as they are loged in. Use webmaster to find out which template the button is in