Just a recap. Admin is not God.

As long as you think you have all rights, you may be stumped and won't look into the usual. Let's get past the "I'm the only user/admin" and remember that even the admin must obey NTFS permissions.

And you would try a new Admin account. And again the old lesson of the single account can not be forgotten. Keep an admin spare account.

Sorry that I can't guess which permission is off but we must get past the single user/admin hurdle before we can continue as well as getting our spare admin account in place.