The Flickr Widget Plugin has been removed. Unfortunately it breaks a blog if it’s activated before Sidebar Widgets are activated. The blog’s address just displays a blank screen, and it’s not possible for the blog’s owner, or a WordPress administrator, to fix it – it’s a system admin job.
Because that’s the last thing we need in a classroom situation, I’ve removed it meantime. Keeping the blog system robust and reliable must take priority. Sorry if you’ve been using it and can’t now.