Public URL for CiviMail: access denied even for admin
Sounds as if you may the ‘Hashed Mailing URL's’ option selected in your CiviMail Component Settings –

Administer > CiviMail > CiviMail Component Settings

/civicrm/admin/setting/preferences/mailing?reset=1

If enabled, a randomized hash key will be used to reference the mailing URL in the mailing.viewUrl token, instead of the mailing ID