The Activity stream doesn’t show anything. It shows a blue button telling me to log in or sign up, and a little text block saying to check to make sure my third party cookies are enabled. I am logged in and have checked that cookies are enabled (they are).
Just delete all cookies in your browser (at least the one from HubSpot). It will then work again.
And remember that HubSpot works best on Chrome. Don’t use another browser as you may encounter some problems (eg. some buttons don’t show or are not working).