Thanks for this report. Can you tell us a little more about what's happening and the circumstance - for example, are you trying to sign in to your account? Or click on a link from an email? Or something else?
A screenshot - of your whole browser screen - showing us exactly what you see when you try to do as you describe (above) would be really useful - thanks!
If you could also confirm your StumbleUpon username, that would also be helpful in investigating your report.