That's because of missing form keys in the templates which you're using in your theme. You should update all phtml files from your custom theme by adding:
<?php echo $this->getBlockHtml('formkey'); ?>immediately after <form ...> tag.
That's the only way.
If this response was helpful to you, consider giving kudos to this post.
If this response solved your problem, click accept as solution to help others solve this issue
