Magento Enterprise 1.x
Logging in to the Magento frontend customer login page (/customer/account/login/) fails. The form submits, but the page simply reloads and clears email/password fields. This is common when copying a site from one domain to another e.g. production domain to development domain.
Check config value web/cookie/cookie_domain in core_config_data table. Update to match current domain or set to NULL (as needed).