I am getting denied access when I try to edit the superuser author record

Hello, I am getting a "denied access" when I try to modify author superuser, even I am accessing my joomla backend as superuser. I attach screenshot.

I am using as security extension Akeeba Admin Tools but I am reviewing my config and I think that no limitations is configured.

May you check at my website what is the problem?

Thank you,

Jacobo