I haven't had that problem and I don't know if I'm being helpful, but I suspect that it's something with the browser. On other websites requiring login which unintentionally automatically log you off, you can delete your history and cookies, restart the browser, and log in again. Once again, this may be of no help to you. XD
As Limxzero said, clearing your history and cookies will generally help. This is a flaw in the browser itself. I believe it only affects those who checkmark the "Remember Me" box.