I'm having the same problem here as well.
I wouldn't be suprised if the problem is caused by changes to the gmail login procedure without them updating their Gmail Notifier software. In some places gmail requests you type a random string of letters as they are displayed in an image, perhaps the notifier is being confused by this request.
Did a bit of searching, didn't find a straightforward solution. A lot of people seem to be having this problem as well.
Please post if anyone finds anything.