Cannot save changes made to Parental Controls profile

I have Parental Controls for one of the users on my Mac. It only allows web access to a list of websites and blocks all others. Recently, however, it has been switched to "Try to limit access to adult websites". When I switch this back to "Allow access to only these websites" and then close the preference pane, when I go back to it, the changes I've made are not saved. No changes that I make to the Parental Controls profile are saved upon simply exiting the pane and going back to it.

I figured it was a permissions error but fixing permissions did not solve the problem. I've removed the com.apple.mcx.plist file from the user folder, removed the user folder from the Library/Managed Preferences folder and neither of these things has changed anything. I'm at a loss here on how to fix it. Thank you for your time.

Solution 1:

The following steps seem to have resolved this issue for me. (Sadly I don't know if this will help anyone else, since the whole Parental controls system seems very complex and buggy, and I don't really understand what is happening here. But I thought I'd note it here anyway.)

1. Delete all unnecessary user accounts (created during testing).
2. On all remaining accounts, tick "Allow user to administer this computer". (Here, I'm trying to ensure Parental Controls is disabled on all accounts and hopefully cause some kind of reset, since I'd noticed that a new managed account I'd created had the same issue - and so the issue wasn't with the individual accounts, it was with the entire machine.)
3. Reboot.
4. On the account that needs to managed, untick "Allow user to administer this computer". (I found it immediately turned back on again: some Mac wrestling may be required.)
5. Quit System Preferences.
6. Reopen System Preferences and check the "administer this computer" change has stuck.
7. Press the Show All button in System Preferences.
8. Cross fingers, wave a chicken in the air, etc.
9. Open the Parental Controls preference pane.
10. Set up Parental Controls again. For me, the settings then stuck and worked.