Magento checkout page redirect to cart after filling information in magento.

I f you getting this kind of error in your magento checkout pages than the problem is on your onepagecontroller.php file


Step1:- open your onepagecontroller.php file

Path:- /app/code/core/Mage/Checkout/controllers/onepagecontroller.php

and than go to line No. 194 or search progressAction method

you see this :- 
 public function progressAction()
    {
        if ($this->_expireAjax()) {
            return;
        }
        $this->loadLayout(false);
        $this->renderLayout();
    }

now replace this method to below.

public function progressAction()
    {
       
        $prevStep = $this->getRequest()->getParam('prevStep', false); 
        if ($this->_expireAjax() || !$prevStep) {
            return null;
        }

        $layout = $this->getLayout();
        $update = $layout->getUpdate();
        /* Load the block belonging to the current step*/
        $update->load('checkout_onepage_progress_' . $prevStep);
        $layout->generateXml();
        $layout->generateBlocks();
        $output = $layout->getOutput();
        $this->getResponse()->setBody($output);
        return $output;
    }