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Users reported not being able to login. This has been traced (in one case) to YSlow loading the page in the background causing the nonce to be regenerated on the server.
A similar issue was observed with Firebug activated.
(In ) fixes #1236 - handle browser/extension/add-on causing a double page load, invalidating the token in the form; refactor Nonce class out of Piwik.php