Ohh yes, how could forget that, this has been asked couple more times in the forum as well. But weirdly, it happens rarely, not in all website. The solution for this problem is to change the form action URL
from: index.php
to: <?php echo JRoute::_('index.php?option=com_virtuemart&view=cart'); ?>
Or simply to: <?php echo JRoute::_('/cart'); ?> (or what ever SEF URL your joomla is generating for cart page)
Be aware: If you make it /cart, and disable SEF URL, then it wont work, but the first one will work all the time.
Regards,
Support