Invalid Form Key. Please refresh the page. error on product save

There is one more source for this error: a wrong cookie_domain in the configuration. As you can't login into the admin area, you can check it directly via mysql:

SELECT * FROM core_config_data WHERE path LIKE '%cookie_domain%';

You can change the value in mysql directly as well. Don't forget to clear the cache afterwards.

The problem was the server configuration, i've change the max_input_vars parameter and the message dissapeared. I saw the solution in http://www.webcroppers.com/magento-invalid-form-key-please-refresh-the-page/

First try disabling any custom modules that you have installed and see if the problem persists.

It looks like something is injecting an extra field in to the product page which is colliding with the form_key parameter which is used by the media uploader and and other parts of the product edit page as a nonce.