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Just ooout of curiosity, are you using a firewall? Could it be that some setting was changed (or activated) on the firewall rather than this being due to a change in settings? Do you have cookies enabled? Firewalls and cookies not being enabled, if I remember correctly, can cause this kind of issue. You are not being recognized, therefore cannot go past this point. I aam probably completely wrong, though.
Someone else will probably have a better thought, but saw that no one had responded so thought I would take a stab at it.