by Juan Leyva.
Hi Sebastián,
somehow, in your Moodle site there is an incorrect value for this setting: guestloginbutton
I'm not sure how this could happen, but there is a simple way to fix it, just use the Site administration search option, search for "guestloginbutton" change the value of the field (from show to hide or hide to show), save the changes and then set the field back to its original value and save the changes.
This should fix the issue because a 0 or 1 will be saved
Regards, Juan