Hello,
Have you tried setting the reply address for the same mailbox when logged in is Adaxes default service administrator (the user that you specified when installing Adaxes)? Such a user has the permissions to perform any operation in Adaxes, and no security checks are performed when logged in as this user.
Also, have you disabled the Automatically update e-mail addresses based on e-mail address policy option for the mailbox? When this option is enabled, the reply address is set based on the e-mail address policy applied to the mailbox and cannot be changed manually.