When I open Administrator mailbox by owa, I got the error:

"A problem occurred while you were trying to use your mailbox".

Insufficient access rights to perform the operation. Active Directory response:00002098

When I try to disable the Administrator mailbox, I got the error:

Action 'Disable' could not be performed on object 'Administrator'.

Solution:

Active Directory Users and Computers

View--Advanced Features

Right click 'Administrator'--properties

Security tab--Advanced button--Select 'Include inheritable permissions from this object's parent.