Problems with the login of a second user

Hello and thanks for your time in advance!

We have got a My Cloud Mirror at home with 3 Logins: Admin, Me, my wife.
We got one shared folder and each of us got one private folder.
Up to about 6 months it all worked fine: the admin couldn’t enter the private folders and we could just access the shared and our own folders.
Recently my wife was denied access to her folder from her laptop, whereas I suddenly could access all folders from my desktop. We canjged her Password - no effect. We then reset the MCM (I think) and it worked for a short time. Now we are back to the start, where my wife cannot acces anythink, but I can.
How can we change everything back to normal?