Unable to change parental control settings

Hi all,

When I try to change the parental controls for a limited account (for the other users of my Dell PC) I get the following error (in the image)

I'm using the only admin account, and the controls worked fine a few days ago (Windows update causes it, maybe?)

Any ideas how I can fix it? The other account can access EVERYTHING at the moment, which I don't want.

Thanks in advance :)