I had this issue, and here’s what was happening to my site. Once a customer got to the checkout page and logged in as a returning customer, spinning wheels got stuck over the credit card (Stripe) area with the same error as described by @nic0net.
In a single site build, a solution on another thread was to remove these lines from your wp-config file:
define( ‘ADMIN_COOKIE_PATH’, ‘/’ );
define( ‘COOKIE_DOMAIN’, ” );
define( ‘COOKIEPATH’, ” );
define( ‘SITECOOKIEPATH’, ” );
In my case, I received the checkout error on my multisite build. I need some of those lines to prevent an admin redirect loop. After some trial and error, keeping these lines solved the problem and allowed admin login:
define(‘ADMIN_COOKIE_PATH’, ‘/’);
define(‘COOKIE_DOMAIN’, false);
define(‘SITECOOKIEPATH’, ”);
Removing define(‘COOKIEPATH’, ”); makes moving from one site to another in the admin break a bit – possibly due to the domain mapping plugin is setup. Logging into a child site on another browser is my workaround until the cookie issues are sorted out.
