[Zope] Adding new Zope Users no longer works!

When we first installed Zope, adding a user who was set up as a 'manager' for a particular folder worked fine. However, recently this process has suddenly stopped working, regardless of the name, password, domain or responsibility levels involved. We can create a new user fine within the relevant acl_user folder, but when we try and log in as that person, Zope complains (after three attempts) that the username and password are incorrect, when they aren't. Help!?!