Try to activate the hidden, built-in Administrator Account (probably in Safe Mode) and see if that works. As it requires an Administrator to do it, it may not work (but since all you'd need is to access/authorize as an administrator as opposed to login as one it's worth a try). Administrator Account
. Use this account to make the changes you need, fix the other Administrator Account, create another Administrator account (for emergencies), and do the suggested procedures. When done, deactivate the account until it is needed again.
I hope this helps.