Your first stop for login issues should be here:
A reader writes:
I'm getting the following error message: "The host you are trying to send the input from is not a valid host."
This issue may affect some users with Norton Firewall software installed.
To get rid of the problem:
Open Norton Personal Firewall (It's usually a globe/brickwall icon in the task tray).
Click on Privacy Control.
Click on Custom Level.
Remove the check-mark next to "Enable Browser Privacy", then click OK.
Close the Firewall config.
This should resolve the problem.