When you sign in normally, you see two empty boxes (username, and password) and a button underneath that says "sign in" right? (Or something like that, I'm not actually looking at the signin screen).

On the flaky computer, you see the button but there are no boxes for entry, right? And if you just hit the button you get the same error message as you would get if you hit the button on a "working" computer without filling in the boxes?

Those entry boxes are a type of web document called a "form" and maybe possibly the ability to process forms is disabled on that computer. Did you say it was a public computer? Maybe try to go to any other website that asks you to register or something... not to fill it in but to see if the boxes are visible and active, if you can type on them. If not, then that may be disabled, or there is something screwed up on that computer that won't let it display forms.

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