I think their server has probably fallen over or something, it was fine two days when I downloaded a copy.
Their github page is still alive (although last commit to master was on the 31st of March).
I never checked their twitter beforehand so I don’t know if a lack of activity in the last 2 years is unusual or not.
Their forums have activity in the last hour but you’re right about not being able to register a new account. I suspect the front-end is working but the back-end is broken or something. (At the very least I suspect forum.fritzing.org and fritzing.org are on different servers.)
I notice the forums page isn’t using https… maybe they’ve been hacked or DDOSed?