Magento Place Order is returning to the cart (basket) page

Hi after upgrading my checkout stopped working and I check each and every possible solution for that, someone is telling that it is due to some other thing etc. etc.

But to know your exact problem first check the javascript errors and then follow this instructions:

Open page "Magentoroot/app/code/core/Mage/Checkout/controllers/OnepageContoller.php"

Search for " $result['error_messages'] "

override this variable with

$result['error_messages'] = $e->__toString();

It will show you the exact problem of what your magento causing.

Hope this helps.