Hm, but you told that previously it worked? Let's get back to the beginning:
1) Make sure *nothing* is checked or entered under Allowed Services, Allowed Sites, Allowed IPs.
2) Make sure that under Blocked Sites the checkbox "Block websites listed here" is unchecked.
Click Disable - Does it work now? If not, go on:
3) Click Schedule -> Individual User Account schedules and make sure all tabs are unchecked (no individual schedule is set up)
Try to disable. Does it work now when there is nothing in any allowed list and that there are no individual schedules set up?
If so, we can proceed to adding or turning on allowed items one by one, after each change testing whether the disable works.