I use dropbox occasionally. I have a free account, one I've had for years. This morning I logged in to access some files a friend shared w/me & dropbox will not let me access it unless I upgrade to some "plan". That page is the same page with which I was greeted upon logging in. I'm at about 650 mg of my 2G free account limit. I am not over storage, unless my free account has been somehow restricted/modified.
I've tried several times to access the notification to "join folder" from my friend, only to be taken to this prompt: