OK so I have an update.
I manually enabled all the users I mentioned (60 in total) - so when I view their profile it shows their plan, with an expiry date in the past.
The users list went out (I couldn't prevent this happening) and I already have one user who logged in (using the username/password provided) and got the message again.
I log into the back end, and that user has become disabled. This happens as soon as they try to log in (I checked it for myself with their account, having enabled it).
Please advise what I am doing wrong as I have followed all the instructions from the documentation and I am at a loss to know why they are becoming blocked when they try to log in - I can provide login etc to get this fixed asap!
Ruth