I think you're the latest victim of the fact that I'm still learning the tikiwiki software, and particularly that I'm still trying to get a handle on the concept of using groups to manage people. When you registered automatically, you were automatically added to the "Registered" group, which provides you with basic access privileges. But there's a second group to which I have to add you by hand, "Members", which confers greater privileges including blogging, and I'm perennially behind in adding new registrants to "Members".

I think I have to change the design of the membership system, and I'm still working it out. Sorry you got caught in the experiment. The advice you got from everybody is good defensive advice, and I recommend following it.