Invites send but don't work

During the set up and tweaking stage I added users (real and dummies) via Admin/User Admin/Click to add new user and all was fine. Tonight I sent out a couple of invites via Tools/Friends/Invite Friends. The emails sent just fine and the recipients received them but they both said nothing happened. I sent an invite to myself to double check and I can confirm it's not working. The email contains the correct text and a clickable link that looks like this: etc, etc.

That takes me to the login page but that's all. I seem to remember there should be an AJAX alert that says to check for a confirmation email but nothing happens.

Any clues would be appreciated as I can't see anything in this forum that resembles this problem.