Alex, i tested this fix, I have no more error ok, but I still do not have the account name == to fullname and "Automatic profile creation from account name" is checked.
So to clarify. This option is useless if you have the misfortune to add a personalized field on the registration form?
So if I understand correctly it will not serve anyone because nobody will leave the form in its original state.
So:
Case 1: The option "Automatic creation of profile from the name of the account" is checked and no field has been added, it creates the profile, ok, but never nobody will be in this case.
case 2: The option "Automatic creation of profile from the name of the account" is checked but fields have been added, so fullname is already pre-filled with account name and grayed out on the second step of the form.

So you can rename "Automatic creation of the profile of the account" by "Account name is identical to name profile / org" so it makes sense in both cases.

That, in my opinion, is how it should work. Otherwise it is useless .... You are not of my opinion?