I got this error after upgrading to php7.0. running magento enterprise 1.9. I then tried every suggestion out there. Here is how I got it to work:

1. I added error reporting in my index.php, and fixed all the errors being reported.

2. In MySQL table core_config_data, I cleared values for web/cookie_path web/cookie_httponly. Make sure you have the right web/cookie_domain value with your domain (very important).

3. Clear your browser cache and cookies.

I got this error after upgrading to php7.0. running magento enterprise 1.9. I then tried every suggestion out there. Here is how I got it to work:

1. I added error reporting in my index.php, and fixed all the errors being reported.

2. In MySQL table core_config_data, I cleared values for web/cookie_path web/cookie_httponly. Make sure you have the right web/cookie_domain value with your domain (very important).

3. Clear your browser cache and cookies.