team member invitation problems

we use the team membership and usually it works fine. today i (as admin) wanted to invite a new member and the invitation was send to the right email address. but the colleague can't accept the invitation. cause the form where you create the adobe ID (with the mailaddress just used for the invitation) is blocked. we can't type in it and so we can't proceed with the registration. i already declined the invitation and send a new one but the problem stays. what else can we do? thanks in advance.