Why can I not access most pages and register on Tumblr? I always get a "Not Found" error - why?

I'm using Google Chrome, have no browser extensions/add-ons, and have Kaspersky anti-virus installed. I always get a "Not found, We couldn't find the page you were looking for" error when I try to register on Tumblr, click "follow," etc. Tumblr support wasn't very helpful. Anyone know what's up?