[Zope] Ghost user in Zope 2.7.6 on Win32

[Dale Strickland-Clark]

We have a strange case of a user disappearing from the acl_users folder. 

The user used to be there and has been using the web site for some months but 
today, she reports that she has lost her permissions. Investigation revealed 
that her user id had disappeared from acl_users (it was the only one in 
there) at that folder level.

Using the ZMI, I added her back in and it all seemed OK but then I realised I 
had given her the wrong role. I went back and corrected the role but she 
disappeared again.

Now, even though the folder is showing "There are no users defined", when I 
try and add her back in, I get "A user with the specified name already 
exists".

I experimented a bit more.

I added another user and gave it Manager role. I can't delete this one.

I added another user and gave it one of our roles, Reviewer (the same as the 
original ghost user). This one vanished immediately.

Can anyone offer any clues as to what might be happening here? Is it safe to 
delete and re-create the acl_users folder?

Thanks for any help.
-- 
Dale Strickland-Clark
Riverhall Systems Ltd - www.riverhall.co.uk