Your first stop for login issues should be here:

http://www.hairtell.com/ubbthreads/faq.php?Cat=0#help

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.