Magneto - customer can't login, can't update quantity of cart items

Here is the solution:

1 – Go to template/customer/form/login.phtml and template/persistent/customer/form/login.phtml and under

add the following code in the login form

If you are using the login form in different templates then you use add the form_key to the other parts as well.

In your theme directory

In your /app/design/frontend/yourthemepackage/default/template/checkout/cart.phtml file or /app/design/frontend/default/yourtheme/template/checkout/cart.phtml file

Just place on line 50 just after getUrl('checkout/cart/updatePost') ?>" method="post"> paste the below code

php echo $this->getBlockHtml('formkey'); ?>